I think the only spits photos you will see with four 20mm in the E wing are those that were used to fly to Malta. They put the extra cannon in so they could be taken out and used on other planes on the beleaguered island. I think the RAF would have loved to put 4x20mm on the spit variants earlier on but it was just not capable enough until the Griffon marks came along with more power.
Actually, the C wing could handle 4x20mm or 2x20mm plus 4x303 or just 2x20mm.
The problem was not with the spitfire but rather with the cannon, which tended to jam-especially in the dust of North Africa and Malta. So the 303s were kept in the wing to insure it would always have some firepower.There would not be much difference between the weight of 4x 20 and 2x20 plus 4x303. I read a report (and will try to find a link for it) that showed no difference in performance. The dust of N Africa required huge Voke air filters and these impacted performance.
It would be interesting to have all 3 variants available- but failing that, a version with 2x20 only would be a fun ride for the more skilled spit drivers.