Hi guys.
If you only have a 15 amp fuse for the load, then thats your maximum load (amps) the horns can draw. A relay rated higher than 15amps can be used, but for the low cost go with a 40 or 50 amp bosch style relay for under $10. They have more contact surface on the points to prevent arching and pitting when switch on and off.. Or at least that what dad use to tell me.
Nice schematic of the horn circuit posted...

but that using a car style horn relay which will needs to be rated for car like the ones the 4 horns are off of, or install 2 relay and have 2 horns on each. Most cars found have a low and high pitch horn.
Wish I had room for the 4 horn set up... sends a message.. WAKE UP...
John