Thanks a lot for the compliments.
I decided to name the Mk IV that because it was such a redesign that I figured it warranted it's own number. The B in Mk IVB is because the Mk IVA has it's guns like the Mk I, close to the cockpit on the hardpoints. I decided to move them out on the wings like the Mk VII and the IVB was born. I have the de Santo Viper as the Mk III btw. I'm naming them like the RAF/RN does. For example, the two seat Viper Mk I trainer I did is the Viper T.Mk I while the nuclear strike varient of the T is the N.Mk I (based on the Mirage 2000N).
So far, I have a Mk I, II, III, IV, V, and VII, T.Mk I, II, and VII, four varients of the T.Mk I, one of the T.Mk II and VII, a PR and FR.Mk I, a GA.Mk III, a civilian Viper Mk I and others that I'll post later.