If you mean to do it yourself rather than pay someone else, I'd say it's hard but doable.
I haven't replaced the screen on an S4 but not long ago I did on an iPad. It was difficult but I did manage it without major incident. If you can follow instructions (which you should read several times before actually starting), are good with your hands and have experience of delicate electronics, you should be able to do it.
The main problems are the size i.e. tiny and the fact that components are glued together. Having said this, I only did it because I got a screen cheaply that a friend had bought to repair his own iPad and then had his iPad replaced under insurance. Otherwise, I probably would have paid someone else to do it. If you've done 20, it's probably a doddle. If you haven't done any, it's tricky.