How many 4x100 strings do you have, and just checking, the panels are arranged in series? If you only have 2, there's no point in a fuse as there can't be sufficient current to blow the fuse with 2 identical strings of panels. Check your local regulations, but that is likely what they'll say too. If you have more than 2 strings, a fuse on each string is important.
Let's assume your string's Isc (short circuit current) is 5 amps. You'd want at least a 6 amp fuse. The fuse should be rated to not blow on the short circuit current of the string being protected, but should blow on the current from 2 strings. If you could get it, 7 amps would be a good fit IMO but MC4 fuses only come in certain values.