If my current phone is on 4G and I change to an iPhone can it work still?

I have a Sidekick under T-mobile and it is with 4G. I'm planning on getting an iPhone 4 and Jailbrake it do it will run under T-mobile but will my new iPhone be able to work even though I had a Sidekick under 4G?

Added (1). I know it has 3G but I'm just wondering if I can work though.

Added (2). *if it can work.

Yes, you can unlock an iPhone 4 and place the sim card from the Sidekick 4G into the iPhone, but the iPhone 4 does not have 4G. It has 3G. Sorry.