Yeah 4Ghz is possible with them on air I think, as long as you have a good quality heatsink and fan.
Is it worth it? If you're playing games then no, nothing will be maxing out your current processor and you'd be better off upgrading the graphics cards if games aren't running fast enough. Even if you're encoding and using software that uses all four cores, you may only see 10-15% increases (that's what I've read). Temps will drop a little because it's 45nm tech but, £270, I wouldn't say was worth the price.