Remixing BY 3.0 and BY 4.0

Remixing BY 3.0 and BY 4.0

I am attempting to make a 16x16 remix tilest using https://opengameart.org/content/16x16-town-remix which is under BY 3.0 and https://opengameart.org/content/16x16-game-assets under 4.0. Are these licenses compatible? I couldn't find anything on Creative Commons about it. 

I've noticed BY-SA 3.0 has a or "later version" clause but I have not seen anything like that in BY 3.0.

Is it preferred to keep the derivatives in separate files and just use layering to combine them in the application?