@nateo44 While the original poster here said it didn't work for them, the only way I was able to solve this issue is to update xpad from source individually, even after doing a full update of the entire system the usual way. From inside of EmulationStation, choose RetroPie, then: RetroPie Setup, Manage Packages, Manage Driver Packages, xpad, then Update (from source). I'd restart the entire system afterward. Once that's done, go back into EmulationStation, hit Start, choose Configure Input, and go through all the mapping again for each side of your X-Arcade Tankstick. Note that this is with the unit in Mode 4 on a newer Tri-Mode Tankstick unit. Worked for me; YMMV.
No idea if it's unique to a Pi 4, or what the deal is, but it was the only method I could find for getting LT and RT buttons to work properly.