I have owned both (4's first then got 4V's then went back to 4's). I would have to agree with the other responders about running the 4v's for stealth in an closer to ORTF config. You can still run the 4's in somewhat of an XY config but you will probably get less consistent results.
I switched back to mk4's because I now run CMC6xt's mainly, which as with the 4V's also have a slight HF emphisis and I thought the V's with the XT's was a little much.
Also the V's taper off faster on the highend of the frequency range around falling off at around 15Khz vs MK4 falling off at around 40Khz (w/ the XTs). This can allow for extended extended frequency response with 24bit (or 1bit) recording.
I also run M222's which have an extended highend frequency range rated around 30Khz (with NT222DC)
I liked the HF emphisis of the V's with the M222's but I pulled some great tapes with the mk4 > M222 as well.
In regards to the higher than 20KHz frequency ranges, they are beyond most peoples audible range (and I am sure alot of playback equipment) so I guess you could argue why do you need this.
In your situation, with the nbox, the above may not be as applicable, but the caps alone should not drive this decision, as your entire rig can push you one way or the other as in my situation. So you may want to see if you can borrow a pair of both and run comparisons with your equipment in the same night (i.e. set 1 4's set 2 4v's) and let your ears guide you.
Both are great caps and will make solid recordings, so if your going to stealth mainly I'd go for the 4v's, otherwise I'd stick with the 4's (4v's are not worth the extra money). I personnaly would not spend the money on the 5's either because omni is only usable in very specific situations. With the money you save on the 4's vs the 4v's I'd save the money and buy a set of mk21's (usable in more situations than omni's) at a later date.
My 2 cents,
-RC