Can I use my iphone 4 from AT&T when I switch to TMobile?

I currently have an Iphone 4 that I bought at AT&T. I might switch to Tmobile. Will it work there as well?

Yes if you get it unlocked.

Yes call AT&T and ask them for the unlock code you have to pay for its not a lot. Then call tmobile ask them for a SIM card or go to a tmobile store, it free. Ask them to activate it and you're done!