I've run the 21's for a little over two years. I almost picked up a used pair of the 21h's that were on ebay but decided to go with a new pair of 21's instead. I'm really happy with my choice as I find the HF boost to be fatiguing after a while. I really like some of the 4v tapes I've heard, but for everyone that I like, there's another that wears on me a little bit. Part of the problem could be running the 4v's backward in Schoeps ORTF bar (70ยบ/17cm) and not a true ORTF, but its tough to determine a pattern as most are FOB tapes but only some have that fatiguing quality. I'm sure some of it has to do with my PSB speakers, which tend to be on the bright side of neutral.
Thinking a little more outloud about this, I just don't see the need for the HF boost for this capsule as you defintely don't need it when recording onstage, and I don't think you need it FOB. Maybe if you were going to run the 21's OTS, but I don't think that would be my first choice to run that far back...
From the charts, its obvious that the 21's already have a slight boost which begins at the same spot as the 4v's, only not as sharp a rise nor quite as exaggerated.
edit:
mk21
mk21h
mk4
mk4v