Except that you're forgetting the T-Mobile's '4G' is really 3G+ (HSPA) which your 4S can get.
You realize there is also HSPA+ which T-Mobile supports. I think the iPhone 4S was HPSA only, not "+". Off the top of my head(if I remember correctly), you have GPRS, Edge, 3G, HSPA, HSPA+, LTE, and then LTE Advance. Depending on where you live, T-mobile's HSPA+ is as good as AT&T's LTE.
As someone said earlier, the iPhone 4S does not support the 3G radio spectrum that tmo uses. So there's no way that the iPhone 4S will deliver 3G/4G HSPDA or HSPA+ speeds on tmo. For the iPhone 5 and 5C/5S, Apple gave it a new set of radios that will work on tmo allocated frequencies, so you will get LTE and HSPA+ speeds on your iPhone, provided that you purchase the iPhone model that supports those frequencies. Not all of them do.
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Except that you're forgetting the T-Mobile's '4G' is really 3G+ (HSPA) which your 4S can get.
You realize there is also HSPA+ which T-Mobile supports. I think the iPhone 4S was HPSA only, not "+". Off the top of my head(if I remember correctly), you have GPRS, Edge, 3G, HSPA, HSPA+, LTE, and then LTE Advance. Depending on where you live, T-mobile's HSPA+ is as good as AT&T's LTE.
http://prepaid-phones.t-mobile.com/sim-card?cmpid=WTR_AF_ALLAFFLTS_6QE0IQXZSJY209