[inquisitor]

You shoud be aware that what T-Mobile USA calls 3G and 4G is in fact the very same network using the same radio technology called UMTS, which originally provided data rates of just 0.384 MBit/s but was increased over the time using backward-compatible enhanced data transmission modes called HSPA and HSPA+. At some point T-Mobile decided that, despite everybody else calls UMTS networks "3G" regardless of which HSPA category has been implemented, they should market their network as 4G where it supports 42 MBit/s. Unfortunately they were even ablte to convince the ITU to accept this misnomer.
That said your Galaxy Tablet will work flawlessly on what T-Mobile calls a 4G network, you will simply not get more than 21 MBit/s because your device doesn't support more. Even if it supported those 42 Mbit/s I doubt you would observe a difference as long as you don't start huge downloads in the middle of the night when the local network is uncongested.
The real fourth generation technology which deserves to be called 4G is LTE which at this time is still pretty immature and at this time offers little to no advantage over UMTS. So don't get fooled by the worldwide 4G hype.