I have a V-Type, V6, and V8.
The V-Type, V4, and V6 all use the same circuit cards. The V6 output card is stuffed with a few extra components for the grid networks of two extra tubes.
I would say the V6 is the most desirable due to the extra headroom and fullness. The V6 seems to benefit from some sort of gestalt; as the difference seems larger than expected.
Between the V-Type and V4, I think the V-Type is more desirable (tolex VS rat fur)...assuming the tolex still looks good. If not, I would go with whichever is in the best condition.
Both of my MkI V-Types are running three out of four of the original 12AX7s. The V-Type has a Gold Lion ECC83 B759 and the V6 has a Tube Store Preferred Series 7025. The V-Type is running JJ KT88s and the V6 is running Sovtek KT88s.
I rolled tubes in the V-Type and chose the Gold Lion so the amp sounds less like the V6. The Gold Lion emphasizes the mids, but is more woolly down low. If I did not own the V6, I would have probably left the original tube in place...I normally try to avoid wool. The circuits in this amp are not particularly responsive to different tubes. The changes in tone and response were pretty minor.
Trace V-Type Tube Rolling | TalkBass.com
The V6 is super sweet sounding and produces huge lows that remain clear and focused. IMHO the lows this amp can produce stand out as remarkable.
The V8 has stronger mids than the V6, but doesn't sound as sweet. I like the V6 better at first, but I get tired of it faster. The V8 seems more transparent and I enjoy playing through it longer. The V8 seems more like a rocker than the V6. A bit leaner lows and more growly/grindy. The V6 leans more towards huge cleans.
The V8 originally ran a bunch of Sovtek 12AX7WA and 12AX7WB. These tubes were still in my amp when I bought it. I rolled tubes and chose a range of models that essentially made the V8 sound closer to the V6...this was before I bought the V6 and V-type. I chose a current production Mullard ECC83 for V1, a Gold Lion ECC83 Standard for V2, and JJ ECC803S pretty much everywhere else. The V8 is naturally a bit more woolly than the V6. The new tubes filled out and focused the lows and added a bit more clarity and color in the mids. The Gold Lion is in the OD channel. I think the V8 was more responsive to tube rolling than the V-Type
Trace V8 Tube Rolling | TalkBass.com
The V4 MkII uses the same preamp as the V8, so I would expect them to sound very similar. I believe the V4 MkII uses the same output circuit, but not the same card as the V8. AFAIK, the V4 MkII is the rarest...especially if you are talking about a head. I have never seen a V4 MkII in person and I think V4 MkII combos may have been more common than the heads.
Feel free to ask a question (or two).