Apple iPhone 4S
T-mobile HSPA+
If they make a 4S with support for AWS (UMTS Band IV) (1700Mhz) then yeah, it will be able to get T-mobile HSPA+ network.
Of course, that still doesn't mean (or make it) that T-Mobile will carry it, in which case, you gotta fork out the money.
http://www.slashgear.com/iphone-5-specs-leaked-by-ci ... »
GSM/EDGE (850, 900, 1800, 1900 MHz)
doesnt look like it supports that, so 2g for the 4s 😢
Even if it did, it only does HSPA 14.4, which isn't as fast as the HSPA+ 21 and 42 that T-Mobile generally touts as 4G.