Yep...3 too many cylinders without a doubt!
We're in luck I think. I still have 2 new Viton seals that I used on the Sloper and I've just dug them out. They are 20 x 26 x 4...and having just measured the sprocket boss diameter (22.8mm) and the crankcase hole diameter (23.7mm), these look perfect for the 4G as well. As you can see, I still have some some sort of oil return scroll on the sprocket boss, but I'll machine this down to 20mm and polish up the surface for the seal. Should work fine.
BTW: with such a narrow seal, these do not have the circular spring in them, but I don't see that as a problem.
'28 Model B (future project), '30 Model F, '31 SF31, '35 4F, '38 VB (abandoned project), '39 4G (current project), '46 VH, '48 KG