Can i use the imei from a unlocked iphone 4 to avtivate a new tmobile sim card?

Will it work?

Yes.
- Just call up Tmobile Customer Care (make sure you have the New, unused Tmobile SIM with you)
- Ask for activations (make sure you know what plan you want - i suggest no contract Value plans for your situation- don't let them sell you!)
- just tell them you have an unlocked iphone somewhere in that process
- they ask you all the good stuff (billing address/ssn/other contact number/credit card number/ etc)
- they ask for all the other good stuff (IMEI # of iphone and SIM#)
- wham, bam, thank you ma'am, you are activated

You can do this at a tmobile store too.

PS they all charge you a $35 activation fee unless you do their cheapo prepaid plans

Yes, most often providers will ask you for the IMEI in order to pair it with the new SIM