Put your 3g on ebay, I got £250 for mine about 3 months ago (16gb).
As for 4 vs 3, I was pretty un-impressed when I got the 4....but that said I was massivly impressed when I first got the 3g, and it must be difficult to improve on something so good.
Theres a definate reduction in signal/network service in my area with the 4, and sometimes it just cuts off when showing 2bars. A couple of little bugs which dont really annoy me too much:
when adding pics to mms, sometimes it just closes msg and goes to main screen.
sometimes msg screen doesnt show the back option so I have to resetthe phone.
most annoying is for some reason not all my emails come through...i'd say 20% come through to laptop but not to phone...that one annoys me!
You wont be disapointed though, just dont expect anything MASSIVELY better than the 3g.