T-Mobile is in the process to deploy their LTE network and if I remember correctly, they will be using Band 4 just like AT&T.
In addition to that, they are also "refarming" their 3G network to be compatible with iPhone and other global UMTS 3G phone with 1900 spectrum (this includes their HSPA+)
http://www.pcmag.com/article2/0,2817,2404090,00.asp
This is what T-Mobile is using currently:
- 1900 frequency as their 2G GSM (EDGE)
- AWS (1700 frequency) as their 3G and 3.5G (HSPA+) network. Not many carriers in the world is using AWS as their 3G frequency.
This is what they will have soon:
- 1900 frequency as their 3 and 3.5G (HSPA+) network. This will make their network more compatible with more phones in the US and globally. Includes iPhones.
- AWS (1700 freq) - as their LTE network. This is Band 4 as we know it. This will align their network with AT&T.