Oh, this is neat.
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A pre-E3 screenshot of Smash 4 was found in Smash 3DS's code. As you can see, it is mainly built upon Brawl's assets. It even has Brawl's renders and textures too.
I guess this also a beta of the Punch Out stage as well.
Very interesting. Even the % meters are just slightly modified from the Brawl ones.
Of course, not surprising that Brawl's character renders are used as placeholders.
The ring's layout is close to the finalized version, but there are noticeable texture differences. The timer seems to display hours, something that was changed in the final version. The % meters and timer are somehow on the stage's big screen, probably due to being unfinished, as the final version doesn't have them. Still, it does have some elements that made it to the final version, albeit modified.
A shame that the resolution is low, because some details may have been lost as a result. Mario's model seems to be the one found in Smash 3DS/Wii U.
Overall, this, alongside the file system, serve as proof that Brawl was in fact used as a base for the new game, even though there are plenty of changes to the engine and several other assets.