It is unclear if content licensed CC BY 3.0 can be upgraded to CC BY 4.0. However - strangely - it is clear that:

• CC BY 3.0 licensed content can be laterally upgraded to CC BY-SA 3.0 content, and
• CC BY 4.0 licensed content can be laterally upgraded to CC BY-SA 4.0 content, and
• CC BY-SA 3.0 licensed content can be vertically upgraded to CC BY-SA 4.0 content.

If anyone can link to information that difinitively confirms or refutes the upgradability of CC BY version 3.0 to version 4.0 please share here. Until then, a couple possible solutions to the above-listed scenario:

1. Combine both of the above example works that are under CC BY 3.0 and CC BY 4.0 into a collection under the single license CC BY-SA 4.0. This does not necessarily change the license of any other assets in your project. Only the derivatives of the two sets of assets would become CC BY-SA 4.0, not the code nor any other assets licensed in separate collections.
2. As you said, keep the two assets separate, not derived into new works, but use game engine wizardry to "combine" them on the fly on-screen. It is true that the combination of individual assets by the game code does not generally constitute a new derivative since the assets remain separate in their "natural" inactive state. However, this seems cumbersome to me. There is no moral issue here in my opinion, it just seems like it would be a lot of extra work who's only advantage is avoiding CC BY-SA components in the project (e.g. solution #1). All projects are unique and have their own unique requirements, so if this is your preferred solution, go for it! :)

You can not use BY-SA 3.0 and BY-SA 4.0 because CC is silly in how they did the liscense.

BY-SA 4.0 is more restrictive then BY-SA 3.0

CC-BY-SA 3.0 license is forward compatible with later versions, however BY-SA is not.

Basically long story short CC is lame, has always been lame, and continue to be lame, in my personal opinion :)

Thanks, Botanic.

... though I can't tell if you're confirming or refuting my suggestion. CC BY-SA is forward compatible with later versions but [CC] BY-SA is not? When you say "BY-SA", are you referring to different versions of CC BY-SA or are you referring to CC-BY?

tl;dr: IANAL, but my analysis is that combining CC-BY 3.0 and CC-BY 4.0 assets is possible; however the terms of *both* CC-BY 3.0 and CC-BY-4.0 licenses are applied to the resulting asset. I believe this is consistent with guidance from the Creative Commons: https://wiki.creativecommons.org/wiki/4.0_upgrade_guidelines#Dealing_with_mixed-version_.28e.g..2C_4.0_and_prior_versions.29_content . Practically speaking, this probably just means you need to include the title of the original work(s) when attributing (a good practice anyway), in addition to listing the author, the license, and the URL.

Longer answer: 

Each CC-BY license governs the distribution and creation of adaptations, for works distributed under that license. To my reading, there is nothing in either CC-BY 3.0 or 4.0 that prohibits different parts of a work being subject to different licenses. As a trivial example, nothing could prohibit you from creating an adaptation by combining a CC-BY 3.0 work (asset #1) with a public domain work (asset #2), to create asset #3. The inclusion of the public domain work (asset #2) does not invalidate or change the requirements of CC-BY 3.0 as they apply to asset #1---why would it? Likewise, license #1 does not affect any users' rights to use asset #2, which is in the public domain. Since asset #3 adapts asset #1, you (and any other users) must comply with the terms of CC-BY 3.0 (i.e. credit author #1, no DRM, etc.). Asset #2 does not impose any additional requirements or restrictions on users of asset #3, since it is in the public domain. An appropriate license for asset #3 would be CC-BY 3.0, which preserves author #1's rights, but CC-BY-SA 3.0 could be used as well. 

Extending that logic to your original question, consider consider that there are three parts to your new combined asset (asset #4):

• The original CC-BY 3.0 work (asset #1, license #1)
• The original CC-BY 4.0 work (asset #2, license #2)
• Your contributions, which can be licensed however you want (asset #3, license #3)

The resulting work (asset #4) is an adaptation of asset #1, so you (and anyone else) needs to comply with license #1. Likewise, it is an adaptation of asset #2, so users need to comply with license #2. Finally, you will choose to license your contributions to the artwork however you like. You could choose CC-BY 3.0 or 4.0 (or 1.0 or 2.0 for that matter, but why would you?), and then users would have to comply with that license as well; you could choose OGA-BY; or you could even choose CC0 for your contributions, and impose no additional terms on the user. (Note: if asset #1 or #2 were -BY-SA, you would be required to license your _contributions_ and the resulting adaptation as -BY-SA as well.) Therefore, as long as the user complies with the terms of all 3 licenses when using asset #4, then their use is permissible. So how should asset #4 be licensed? I would argue an appropriate license is (CC-BY 3.0 AND CC-BY 4.0). That is, a license that requires the user to comply with the requirements of BOTH CC-BY 3.0 AND CC-BY 4.0. More on what that means practically below. OpenGameArt does not currently have a way of indicating that terms of multiple licenses MUST ALL be followed (e.g. conjunctive "AND"), only that users may choose from among multiple license options (e.g. disjunctive "OR"). Other groups/systems such as SPDX do allow for this distinction; see their documentation. 

Practically speaking, what does it mean to comply with CC-BY 3.0 and CC-BY 4.0? From the Creative Commons Upgrade Guide for v4.0 :  

In many cases, a work will have multiple rights at play because there is more than one contributor to a work. This often means there are multiple license versions applicable to a work. In those cases, reusers must comply with all relevant license versions when reusing the full work.

For, example, reusing a 4.0-licensed translation of a 3.0-licensed essay would require complying with the conditions of both versions 3.0 and 4.0. This means attributing the original author as specified in v3 and attributing the translator as specified in v4.

... As a practical matter, compliance with a later version of a license is typically compliant with the earlier license. There are certain exceptions with respect to attribution. A comparison of the attribution requirements is here

Basically, if (as a user) you are following the rules of the later license, you are generally following the rules of the earlier version. The major exception is that later versions have different requirements for attribution. As far as I can tell, the only practical difference is that CC-BY 3.0 required you to include the title when attributing an asset, whereas CC-BY 4.0 does not. There are other differences about linking to the license and such, but here complying with CC-BY 4.0 should satisfy the requirements of CC-BY 3.0. See details here: https://wiki.creativecommons.org/wiki/License_Versions#Detailed_attribut... .

I don't know what Botanic means by "You can not use BY-SA 3.0 and BY-SA 4.0 because CC is silly in how they did the liscense." You can combine a CC-BY-SA 3.0 work and a CC-BY-SA 4.0 work. The license of the resulting work is (CC-BY-SA 3.0 AND CC-BY-SA 4.0). All the same analysis above applies to combining a CC-BY-SA 3.0 work with a CC-BY-SA 4.0 work. If anything, the situation is more clear, since the CC-BY-SA 3.0 says you MUST distribute an adaptation under the same license or a later version of the same license (CC-BY is silent on the exact license you can use for adaptations). See here https://opensource.stackexchange.com/questions/7430/can-i-upgrade-the-ve... . See also this question which presents a similar analysis to mine: https://opensource.stackexchange.com/a/6782

I do agree that "CC is silly in how they did the license," insofar as this is very confusing, and it would have been nice to have terms like "use this version or any later version of the license" in CC-BY 1.0, but nobody is perfect, and they are trying to improve the licenses over time to make it easier to use content. 

@z9484: I don't believe that would address the needs of the new remixed asset based on the two assets in question. As bluecarrot16 put it, asset #4 could not be shared on OGA twice; once under BY 3.0 and again under BY 4.0 as that would imply a disjunctive OR of the two terms. In other words, it would be no different than saying "you can use either license you prefer, only one needs to be adhered to." Which is not really true for the situation you've outlined. 

If you're wanting to know how to be faithful to the terms of the licenses for the purposes of your game, but don't feel the need to share "asset #4" here on OGA, then you could list it in your game's credits as CC BY 3.0 AND CC BY 4.0.

However, if you wish to host the new derivative "asset #4" here on OGA, there is no mechanism for indicating that users must adhere to terms of two separate licenses simultaneously, so it would need to be shared under some single unified license... which I am now unsure is an available option without permission from the authors of both asset #1 and #2.

Good news! Although the above discussion is correct that there are limited options available for unifying the licenses of CC-BY 3.0 and CC-BY 4.0 assets, IN THIS PARTICULAR CASE, the derivative in question "asset #4" can be simply licensed CC-BY 4.0. 

This is possible because, although some of the assets are CC-BY 3.0 or are derived from other CC-BY 3.0, those parts of the derivative tree already have a waiver from the authors saying *The authors of this content agree to license it under later versions of the licenses they selected above.

Here is the complete derivative tree for the assets in question:

• z9484's Remix, aka "Asset #4" (CC-BY 4.0) which is a derivative of:
• George_'s 16x16 Game Assets (CC-BY 4.0 though it was previously CC-BY 3.0 up until 10/29/2017) and is also from:
• Sharm's 16x16 town remix (CC-BY 3.0 or OGA-BY 3.0)* which is a derivative of:
• Surt's Town Tiles (CC0) and is also derived from:
• Redshrike's RPG Indoor Tileset: Expansion 1 (CC-BY 3.0, GPL 3.0, GPL 2.0, or OGA-By 3.0)* which is a derivative of:
• Redshrike's 16x16 indoor rpg tileset: the baseline (CC-BY 3.0 or OGA-BY 3.0)* and is also derived from:
• Jetrel's RPG item set (CC0)

Therefore, all assets are either already CC-BY 4.0, are CC0 (which can be relicensed to anything), or are CC-BY 3.0 but the authors have pre-approved relicensing to CC-BY 4.0. Yes? No? Thoughts?

Attribution would probably look something like:

"z9484's 16x16 remix (CC-BY 4.0) based on George_'s 16x16 Game Assets [opengameart.org/content/16x16-game-assets], Sharm's 16x16 town remix [opengameart.org/content/16x16-town-remix], Surt's Town Tiles [opengameart.org/content/town-tiles], Reshrike's RPG Indoor Tileset [opengameart.org/content/rpg-indoor-tileset-expansion-1], and Jetrel's RPG item set [opengameart.org/content/rpg-item-set]"

Hope that helps.