First of all, I'm not into A4000s, but replacing the fan in your PSU shouldn't be that hard. Most likely it is a standard 60 or 80 mm fan, which can be found almost everywhere. Just look what size fits best, buy the appropiate fan and replace the old fan with the new one.
In the worst case (which I expect to happen), you have to cut the old fan's wires.
Take your time, check everything for a second time while working and there's not much that can go wrong.
