Well, as far as I'm aware, the only difference would be that it's wider to accommodate another string. How that would be a benefit to a 4-string, I have no idea...
I don't know about 5ers, but I think that the extended range pickups are wound differently for the lower range and thicker strings. A lot of 4 string players switch to BEAD strings without any modification aside from widening the nut and saddle slots, so that probably isn't the case.
You might want to look more into pickup winding and find if maybe you could get a 4 string pickup wound to the specifications of a 5 string, if there are indeed any differences. That would save you the routing that you'd have to do to for the wider pickup.
