I've been reading posts by developers using the iOS 7 gold master - it seems to run quite smoothly on the iPhone 4S, at least. I'm not as sure about the iPhone 4, because, well, I have a 4S, so that's what I've been reading up on.
I’ve been using iOS 7 on a 4s for the past couple of months. Works great. Started a little buggy, but the first thing you will notice is that it is decidedly faster. The big effort Apple made with this is to flatten out the design elements. Fewer bevels, shadows, fake textures, etc. They were not completely successful, but flat is the new black, so you would expect them to go there.