I know Installous chat gets frowned upon, but if used "responsibly" it can be superb.
Over on Touch Arcade forums there's been two threads running on Sonic 4, many proclaiming the frame rate is awful on iPhone 4 and that it looks awful without Retina Display visuals.
I downloaded Sonic 4 in Installous, played it, loved it, and legitimately bought it from the App Store having tried it for myself and done away with the worries from Touch Arcade's somewhat childish rants.
£5.99 is cheap for a game like Sonic 4, but it's nice to be able to test out a game before leaping at that price, especially with people complaining about nothing more than the odd judder in places.