Amiga 2000 controller port fuse strangeness

Trying to help out a friend, he's unable to get the mouse working on his Amiga 2000. We've determined that the +5V line(s) on the DB-9 controller ports are dead, which would tend to account for the non-working mouse. That is, I assume the mouse electronics draw their power from the +5V pin. So the +5 pin on each of the two ports lead back to a common point, which is a fuse.. thing is, it's a surface-mount fuse, and it's marked 4A.. as in FOUR AMPS. Now the schematics we have do show the fuse, I believe it's indicated as F1. But there's no rating on the diagram. Can anyone confirm that this is actually a 4A fuse? Why on earth would they have such a high-rated fuse protecting those lines? What sort of controller-port peripheral device could ever need that much current - and is a common DB-9 connector (or for that matter, the PC board traces) even capable of that kind of current? 400mA maybe?