There are really two sonic differences between these two capsules. One is the thing that y'all are talking about--the overall response elevation at around 9 - 10 kHz which the MK 4V has and the MK 4 doesn't.
But the MK 4 has a slight elevation in its off-axis high-frequency response which the MK 4V doesn't (in the horizontal plane, at least). So the two capsules have different relationships between their on-axis and off-axis response, or (if you prefer) their free-field versus diffuse-field response. That's what jerryfreak was referring to.
For recording in a large room with rich, warm, well-balanced acoustics, either capsule can be used equally well; they won't sound the same but it's purely a matter of preference. But when there are less-than-desirable reflections which tend to be bright and harsh--as in small performance spaces with hard, nearby walls and/or low ceilings--then I strongly prefer the milder diffuse-field response of the vertical capsule. I may not be in love with its response elevation, but that can be tuned out with a good equalizer--whereas an equalizer can't help you when the high-frequency content of the reverberant sound is out of proportion to the high-frequency content of the direct sound.
In other words it isn't that one capsule is generally "better" than the other--but that the vertical capsule is more suitable for certain imperfect acoustical situations which I've come across rather often in my classical recording work (especially when recording grand pianos, which people insist on putting into rooms that are way too small for the instrument).
For those who don't want to have to choose between a high-frequency elevation off-axis (MK 4) and a high-frequency elevation on-axis (MK 4V), there is a little-known alternate form of the MK 4V called the "MK 4VJ" which doesn't have any high-frequency rise to speak of--just a little "blip" of about 1 dB at the top end before it starts to roll off gradually. I've been unable to convince Schoeps to list it in their catalog, but it can be ordered from any Schoeps dealer.
--best regards
P.S.: added the next morning: The response elevation of the MK 4V isn't a crime against nature, unless you consider the entire recording industry to be a crime against nature (though as someone who's worked in that industry, I'm open to discussing it). The response elevation of the MK 4V is part of what it takes to "sound like a recording" in our day and age. As a classical musician I prefer situations which allow me to record with a fundamentally natural and realistic sound balance, but a great many recording situations don't allow such methods to work effectively for acoustical reasons. Also, many listeners (especially those who aren't musicians) don't expect to hear music coming out of a stereo; they expect to hear a recording as that phenomenon has been defined commercially.
Commercial recording is an idiom or more precisely a set of idioms. An important part of most of these idioms involves the exaggeration--sometimes extreme exaggeration--of certain kinds of sonic detail. Multi-track recording with overdubbing, close-up miking and the use of microphones with non-linear frequency response are an integral part of that approach.