It's been almost a year since I swapped my SGH-I337 (At&t S4) for my SGH-M919 (T-Mobile S4) but I do remember SIM unlocking and enabling the AWS bands on the At&t phone and testing the 4G with a buddy's sim card. Everything worked fine.
Side note: the SGH-M919 works flawlessly on At&t's network. 4G LTE, MMS, visual voicemail, etc. No issues whatsoever. Hope this helps!