The 4021s are great and very easy to stealth.
DPA can shorten/lengthen the cables for you. Their price is pretty good, and it's worry free. I think I paid $30 to have them do mine.
B&H will not bargain, and if DPA is like most high-end manufacturers they won't let people list a lower price. However, I would try Justin at Pro Sound (justin@pro-sound.com I believe) who can sometimes discuss discounts privately with
TS.com members.
They should work fine with the 661.
You could buy used, though since they are discontinued the used sets now are getting older and older. Also, that side exit cable is prone to breaking down over time. If I were you, and I could, I'd buy new from DPA and get mics with a warranty that you know have not been abused.
That said, post an ISO in the YS. It can't hurt.