I didn't play halo 3 so my answer may not exactly be adequate, but I'm guessing that Halo 4 is better.
I stopped playing Halo back when Halo2 came out and it was disgustingly worse than the original. I just recently played Halo4 at my friend's Halo4 release party at a movie theater and it was pretty fantastic. I really enjoy how they went back to the roots. The game play was very similar to the original, except it had many marked improvements.
I know my answer isn't fair, but if Halo3 is anything like Halo2, then Halo4 is definitely the superior game.