I changed the battery in my Mum's iPhone 3.
After watching it being done on Youtube a few times, it ended up being fairly straight forward.
I understand the 4 is a lot more delicate inside, and I don't doubt this. I'd previously dropped the 3 innumerous times (when it was mine) and there was never any damage, but I've seen and heard of heaps of 4s with broken screens, so be careful if you decide to go in.