T-Mobile has started rolling out a new update to Samsung Galaxy S4 and Galaxy Tab 3 units on its network. Arriving as firmware versions M919UVSFQA1 and T217TUVSBQA1, respectively, the update brings Android security fixes for the month of February.
A quick look at the magenta carrier's website reveals that the update to both these devices (which are still on Android 4.4.4) doesn't include anything else. It's also worth mentioning that this is the first update these devices have received since 2015.
As is usually the case with OTA roll outs, it may take some time for the update to hit your device. Meanwhile, if you feel impatient, you can manually check for the update by heading to your handset's Settings menu.