Moto X has 5 models with different LTE bands. Does it matter which one I get?
LTE bands are the bands of radio spectrum that the cellular networks run on.  There are 44 defined bands and different carriers in different areas use different bands to provide service.  Wikipedia has a fairly extensive list of who uses what bands.
Yes, T-Mobile operates on band 4 (aka AWS), so any other band capability on the device would go unused.
If you don't have the same band available on the phone as on the network, you won't be able to use the network period.  It would be like trying to pick up an AM radio station on an FM-only radio.
Yes.  Band availability is determined by the hardware radio set the phone was built with.