Note: I was able to get it working fine on my T-Mobile Galaxy S4 but was having some reboot/touchscreen issues at random times, not sure if this is an issue with my S4 but if it happens to you, it must be the ROM. But works 100% flawless on my GT-i9505 S4. This ROM may also work on other variants which I am still testing out and should have an update on that too.
Download Thor S4 ROM for Galaxy S4 (for GT-i9505, T-Mobile SGH-M919, Canadian SGH-i337M)
For fail-proof installation, see How to Install Android Marshmallow ROM using TWRP!
To install, reboot into recovery, make backup ROM, format /system, wipe data/factory reset, install ROM. (If for some reason you end up in a bootloop or get force-closed keyboard error on boot, reboot into CWM recovery, go to Mounts & Storage->format /system. For TWRP, go to Wipe->Advanced Wipe->Wipe /system.)
If you are having call issues or other issues, try installing the “Fixes” which has several fixes for bugs.
Q: My 3G/4G LTE data is not working!!!
A: See How to Fix 3G/4G LTE data by Manually Setting APN on Android!
What is a Custom ROM?
A custom ROM is a custom, modified operating system that you can install on your Android device. Basically it's like installing Windows OS on a Macbook. By installing a custom ROM, you can bypass stock factory settings like locked hotspot tether, install new Android versions early, or even convert your phone into another OS.
With a custom ROM, you can really maximize the capabilities of your smartphone by adding custom features that have been developed by open source developers.
How to Install Custom ROMs
First, you will need to root your device, if it is not rooted yet, follow our root tutorial and root your phone.
Once rooted, you should have TWRP installed on your phone. Installation of a custom ROM is easy if you learn the basic of it. If you don't know how to install custom ROMs using TWRP recovery, see our tutorial on How to Install Custom ROM using TWRP!