How does the horn get return power? If the one relay wire goes to the horn, where is the other other side of the horn connected? My guess is that the horn has only one lead, which is positive, and the horn body is grounded when you attach it to the frame.
If so, then the relay other side is also probably grounded to complete the circuit.
To be safe, I would use a 1.5 amp in line fuse.
Fourth wire goes to ground and the positive should be fused.
I just bought the hella horn set from moss that comes with a relay. Unfortunately the instructions are poor. The relay has four terminals. One goes to the horn switch wire, on goes to the actual horns positive side, one goes to the power/battery. What does
the fourth terminal connect to? In addition the connection to the batter/power requires an in line fuse. Any idea what size fuse?