These relays comes with 30 amp fuse. Headlights are 55w, so both together will draw 9 amp. A 15 amp fuse would be best.
The fuse is to protect the wiring and the headlamp wiring should be capable of withstanding an over-current for as long as it takes to blow a 30 amp fuse without getting hot. If you set the rating too low you will have problems with the surge when you switch the lights on or when a bulb fails blowing the fuse.
There are 3 relays for dipped beam, high beam and fog light.
What for is the fourth relays?
According to my service manual, it is for the electric windows.
Incidentally, in my formative motoring years, it was very rare to have fuses in lamp circuits, the theory being that if the fuse failed, the driver could be unexpectedly plunged into darkness at speed. I used to own an Austin Cambridge and a Morris Oxford, both of which had only two fuses, to protect auxiliary equipment and I believe Fords of the same period had no fuses at all.