Only one squad of Mk Vs used four 20mm cannon. It would be horrible overused in AH. Bad idea.
Spitfire F.22 was post war. The Spitfire F.21 saw a very limited amount of service. There are much higher priorities than the Spitfire F.21.
Might not be 100% accurate.
Some of the infamous Malta Spits also left the carriers with 4x20mm, once on Malta 2 were removed.
Unknown if any were inbound and got caught up in any ongoing dogfights, guess it's not beyond the realms of possibilty.
Apart from that the 'old' AH Mk Vc (2x20mm+ 4x.303in) definately needs to make a re-appearence.
Only this time clip it, but retain the cropped impellor and boost settings (Merlin 55M).
F.21, yup for a later date, needs parts remodelling (wings for e.g.)
However all the parts should be available for a Mk XII.
Those 3 Spits would effectively cover all the major/important wartime variants with what we already have -
I, F Vb, LF Vc, F IX, VIII, XII, F XIV, XVI, F.21
A simple change of an option on the XVI for 2x20mm + 4x.303in makes it a 1943 LF IX (2 for 1 effectively)