You can see the HF push Senor Nizzel mentions in the frequency response plots below. The slight HF push will make them just a touch brighter, and some people like the slight HF push to compensate for the loss of HF energy when recording at the distances - and in the environments - we do. Personally, I don't think they're worth the extra cash and was happy for a long time with my MK4s (before I traded them in / bought MK5s, switchable omni / cardioids). IMO, if you're gonna spend the cash on MK4Vs, might as well go for the MK5s and get the omni, taboot:
$ 600/ea | MK4 (cardioid)
$ 985/ea | MK4V (cardioid)
$1,048/ea | MK5 (switchable omni/cardioid)The MK5s share a slight HF push with the MK4Vs, but not quite as strong a push, and broader in range.
If you're going to buy new, do it soon - prices go up ~8% on Apr 1, I believe (per a thread in the Retail forum).
Dunno if that answers your question, but... $0.02
MK4
MK4V
MK5