The main reason the four 20mm option saw little use was because the guns used to jam at alt. The Spit wing didn't have enough hot air piping to supply pairs of 20mm. The weight also screwed the plane's performance to a degree. That's why they were only used for ground attack, and even then only very rarely. If AH modelled gun jams I would feel happier about having this as an option.
Perhaps when AH's long awaited perk ordnance system finally appears.