If I update my Iphone 4, will my phone be locked?

When I bought the phone, it bought it at T-mobile and it was already unlocked. So will it be locked when i update. Also, what happens when it is locked? Is the phone still useable? Apps, music, etc

Nope, it's just a software update. If the phone is going to locked, you can't use it unless if you're going to bring it to a cellshop to unlock it again.

It won't

No, it will not be locked and all your stuff will still be usable and accessible. Software updates have no effect on the sim card.