Apple iPhone 4S
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.