Thinking about it. I think Smash 4 potentially has the ability to make Brawl obsolete. The greatest thing I loved about Brawl was the character and stage selection. If Smash 4 brings back all of Brawl's roster (except possibly Snake and Toon Link), then I would have pretty much as much incentive to go back to Brawl as I did with Smash 64 after Melee's release.
However, even if Mewtwo and Roy return for Smash 4, I would probably continue to play Melee. That game is timeless even its roster was lacking in comparison to Brawl and potentially Smash 4.
Personally, I like Brawl as much as Melee, however, if Smash 4 brings in all the stuff I loved about Brawl without any of the downpoints of it, Smash 4 is going to make that game seem obsolete.