Originally posted by Krusty
But in practice it didn't really happen that way. There were also problems warming the outboard guns at altitude, if memory serves. Most that had the 4x gun setup had 2 removed because "2 were enough" -- at this time London was no longer under seige and the bulk of the targets were Luftwaffe fighters, smaller aircraft than the big bombers of the BOB.
EDIT: Got a picture of that spit? That'd be kinda cool to see.
Should be noted it was only a cannon mock up. (looks evil though)
DP845 Prototype Griffon Spitfire Mk IV
There were also a handful of RAAF Mk VIIIs with 4 x 20mm also.
Problem is going to be exactly the same as the clipped wing argument - 4 x 20mm Spits existed, just how many is unknown.
Yes there was a problem with the Mk IX and heating for the outer pair, probably why most fitted with quad 20mms are found on older Mk V's in warmer climates.
[edit] This of course does not include the F.21 (if we ever get it), ALL of which had 4 x 20mm fitted as standard, in a re-designed wing.