An "LTE band", or any other type of "band" that you read about with relation to a phone, is referring to the radio frequencies that the phone picks up. Each cellular provider builds their network on a specific set of radio frequencies which are divided up into chunks that are assigned band numbers. So, using your T-Mobile example, band 4 is specifically comprised of two frequency ranges:
- Upload bandwidth uses radio frequencies between 1710 and 1755 MHz
- Download bandwidth uses frequencies between 2110 and 2155 MHz
A phone's spec sheet effectively lists the frequencies that its antennas (and cellular radio) are designed to support. You cannot change the antenna's supported bands because this is not something that is set in software - the supported bands listed in a spec sheet are, generally, a statement about the phone's hardware support for different networks*. Therefore, your phone must support the carrier's LTE network bands in order for you to have LTE coverage. Otherwise the LTE radio will be unusable.
Cellular antennas these days are pretty much always multi-band pieces of hardware (that is, one antenna picks up several bands). The difference between powering a 5 band radio and a 2 band one is probably measurable with very precise instrumentation, but it's certainly not going to be an amount that would be perceptible when actually using the device. Further, having support for additional bands is generally considered a net benefit because you have the ability to use the phone on multiple carriers (assuming a SIM unlocked device).
Per your specific example of the XT1053 vs the XT1055: you could use the either on T-Mobile USA's LTE network, because both support band 4 LTE, which is what T-Mobile is using. However, the XT1055 will not work on T-Mobile's current HSPA+ network because it lacks support for the UMTS 1700 band. In places where LTE is too weak or not available, you would drop all the way back to EDGE (2G) speeds, whereas the XT1053 would be able pick up HSPA+42 speeds (if available).
*There are some exceptions to this rule, where a device may actually use a radio chipset/antenna combination that provides hardware support for a band that is effectively "locked out" by the software layer. One example of this is the Nexus 4, which does not list LTE support in its specs but does, in fact, have an LTE band 4 chip installed that can be enabled on older baseband versions. However, you could not force the N4 to work on, say, LTE band 17.