I wouldn't say the fan reaction for Colors is a fair comparison with the fan reaction to Sonic 4. I detailed my thoughts on the 2D and 3D series elsewhere on the forums, but basically I see the original 2D series as an excellent platformer whereas the 3D series was an adaptation of the 2D series, with mixed results (I'd call Adventure 1&2 great, Unleashed day stages very good and the rest mediocre to horrible).
Really the 3D series has had so many problems and was at such a low point, that all Sonic Team had to do was to take what fans accepted as good and decent from the previous games, double it and release it as a brief but fun Wii title. Colors doesn't look to break new ground, but it does look like a fun game to complete over a weekend. It also helps that ST threw in a bit of fan service by way of badniks (hey, Sonic 4 does that too!) and Eggman gags.
While Sonic Colors had nowhere to go but up, Sonic 4 is coming off of some of the greatest games of the 16-bit days. I think it's rather silly to assume that 16 years later, SEGA would outdo the original trilogy in a world in which games have evolved well beyond 2D platforming. Really, Sonic 4 has nowhere to go but down, but that is in no way a bad thing. The best Sonic Team can do is bring us a game that is as fun as the old games, with a few new tricks, and a graphical upgrade. Some folks here have labeled that as a "good enough" mentality, but I'd like to inquire what would be more than that?
If you try to outdo the classics both in gameplay and graphics, you risk becoming what the 3D games became. If you match the classics, both in gameplay and graphics, you have something that resembles a good portion of the fan-made games out there (or Mega Man 9 & 10). What I see when I look at Sonic 4 is a good middle of the road solution: It looks like the classics with a touch of the modern in HD.