I asked a wartime Spit pilot why you never saw the Mk IX in four cannon fit. They had the fitting for four cannon but only carried two. Apparently the wing flexed so much when firing four cannon that aiming became pretty much useless.
As has been noted, the early F4 carried only four 50 cal guns. There was some opposition from pilots when they released the six gun wing as the ammunition carried for each gun was less, and as the pilots figured, if you can't hit a target with four guns you're not going to hit it with six.