No, it doesn’t have 4G connectivity. It has HSDPA, which as they describe uses one antenna to send data, one to receive (Think DC vs. AC current), so it’ll be double the data speed it was. It’s not the same as 4G which Verizon, Sprint, and AT&T have along with almost all new Droid phones.
Differences from your 3GS:
Camera is much better on 4 and 4S, along w/ 1080p video capabilities
Display
FaceTime/front camera
Dual-core chip
Will it be an upgrade from the 3GS? Yes, very much so.
Will it be an upgrade from the 4? Not much, but still a little bit.