The limit is there to artificially increase the difficulty of killing a Thargoid.
I suspect it's because if there were no limit AX combat would be dominated by big ships, and newish players would feel there was no point in them trying it, since to the new player the big ships seem to be an impossibly distant goal (we old hands know that's not necessarily true, but then that's one of the things that make us old hands).
Whereas it's not so difficult to get yourself into a ship that can fit four AX weapons, and then you can join in the fun pretty quickly..
if that would be the case,
then they could just release small version of the AX gear.
i mean, they release new anti-thargoid ships (Chief variants), but not even those can really fit a full anti-interceptor set of weapons, without using guardian stuff
chief has no place for the flak
challenger same
crusader - worst of all
so - with the AX weapons alone, the max-4 limit is more or less something you will face only on a handfull medium ships (dropship/gunship, FDL, Krait mk2, python)
with the introduction of small guardian modules the thing became an issue.