Ive never heard of a gas sending unit to go out on an S4 but saw one online that said it needs replacing. is this a common problem? How hard would this be to fix and is it drivable until it fixed?

Just have never heard of this going out on an S4

 

the feul pumps should last a long time. You just have to make sure your not always going on E all the time because they gas keeps the pumps cool.