What is the best sounding DAC at $2000-$4000?

I have never heard one but the new PS audio perfect wave II from what I've read sounds like it would definitely be worth checking out. Beyond that my best sound has come from a bryston BDA 1. The benchmark DAC for much cheaper isn't bad at all either and is a super deal at around 600 used.

No one knows as no one has ever bought more than a few of the dozens of DACs available. Asking your question is like asking for a miracle.
If ANYONE claims they know the answer, they are either confusing what they bought as having to be the best because, naturally it is the one they bought, or because they are just being a shill for some brand either they like, or have a vested interest in.
If you what at least a reasonable chance of any sort of answer, read reviews of DACS by Googling the brands and reviews.
Asking here is a total crap-shoot.

PS: USB still seems to be a bit flaky, and maybe in the next year or so some may arrive with the sort of performance you want. As is, there is only one well regarded USB device which was reviewed in Stereophile. Since then, several companies have better (than their prior) USB implimentations in the works..
Hang in there..

The Ayre QB-9 is an excellent choice and fits within your budget.

Have you considered the Ayre QB-9? It fits your spec, and is very highly recommended and reviewed. I really enjoy mine. Heck, I'm enjoying it right now!