T-Mobile won't sell you a Straight Talk sim, no.
Straight Talk uses 4 "real" carriers for their Straight Talk-branded phones: Verizon (CDMA), Sprint (CDMA), AT&T (GSM), and T-Mobile (GSM).
Straight Talk is buying the excess bandwidth of the carriers and re-selling it.
Starting last February, Straight Talk started a revolutionary program: Bring Your Own Phone (BYOP), where you could bring ANY GSM (sim card) phone you own, and they will sell you a Straight Talk sim card to put in there. For $45 per month, you can have service on either the AT&T network or the T-Mobile network. Your choice... BUT you are not an AT&T customer nor a T-Mobile customer. You would still be Straight Talk customer.
You have to use a Straight Talk sim. You order these from www.StraightTalkSIM.com -- a separate website from StraightTalk.com. You will pay $15 for the sim card.
Net10 is ALSO participating in the sim card program. You can order a Net10 sim card from Net10SIM.com.
If you really, really want service on T-Mobile, then choose the "T-mobile" sim. Note I'm putting T-Mobile in quote marks. It's not really from T-Mobile, it's still from Straight Talk, but configured to work on the T-Mobile network through Straight Talk.
I wanted an "AT&T" sim and I knew what size sim I wanted, also. I'll re-use a screenshot I used before, so ignore the markups, it was for someone who had a Galaxy Nexus like me.
One other issue. Are you SURE you want to use T-Mobile network on the iPhone 4S? T-mobile is refarming spectrum (frequencies) they got from AT&T, but until they finish all the "regular" T-Mobile spectrum only allow slow EDGE data speeds. With the newly acquired AT&T frequencies transferred to T-Mobile , you will be able to receive HSPA+ speeds on your iPhone 4S. However, I would check your area to make sure you have or will get the transferred spectrum in your area. Otherwise, you will have painfully slow data speeds on T-Mobile.
Yes, wi-fi speeds will still be great. I'm just talking about data speeds over the mobile network.