I wasn't able to find much info on the G4's radios. But the S6, according to T-Mo's website, has:
- Quad Band GSM; LTE: 1, 2, 3, 4, 5, 7, 8, 12, 17; UMTS: Band I (2100), Band II (1900), Band IV (1700/2100), Band V (850), Band VIII (900)
We should know about the G4 in a few weeks when the post it. For now all websites I found just had general info, not carrier specific.