Do I need to factory unlock and iphone4s to hook it up to Tmobile?

I wanna get a iphone 4s but i have Tmobile. If i get a 4s could i just stick my sim card in there? If not do i have to factory unlock it? And any additional info would be nice to.

It has to be unlocked. Data will be 2G and MMS will be useless.

You will have to make sure it is either factory unlocked or unlocked. MMS won't work (no picture messaging) and you won't have 3G/4G, just standard Edge 2G data. You are better to buy a smartphone on TMO.

Yes you need get it unlocked.

If you buy a network locked iPhone 4S, You need to unlock it from network restriction to use it on T-Mobile network. You can unlock an iPhone 4S safer and easier using remote unlocking service. This method of unlocking is safe, simple, affordable and quick. You can get the remote unlocking service for an iPhone 4S from http://www.wickedunlock.com/apple-iphone-4s/rs16wp9/ at affordable cost with easy step by step unlocking instructions. This method of unlocking doesn't require a jailbreak too. All the best mate.

If your iphone is AT&T we can unlock it without jailbreaking it. Because this unlock is official and does not require a "jailbreak", it means you can safely update the OS on your handset at any time, and sync your device with your computer without fear of Apple removing your unlock
You tell us your IMEI, Having made payment, we then mark your IMEI as unlocked within Apple's IMEI database - once this is done we send you an email confirming this. You then simply connect your iPhone to iTunes in order to complete the unlock. Your unlock is guaranteed and is also permanent, meaning you can safely update your handset and sync with iTunes without ever worrying about it re-locking.http://attiphoneunlocker.com/
Your phone can now be used with any gsm network.
1) Open iTunes on your Mac or PC (verify you have internet connectivity).
2) Connect your iPhone to your computer using the USB data cable.
3) Backup and Restore your iPhone using iTunes.
4) Once your iPhone is restored it will have the Factory Unlock in place!
http://attiphoneunlocker.com/faq/