Two years ago Eric Shelton was running OSX in Qubes: you will find his
mails in the list archives.
I was able to boot a hackintosh image that I had following his approach,
but the performance was not good, and I had little interest in making
it work. But yes, in principle, (and no doubt after much hacking about),
you should be able to run OSX in Qubes.
I've had good experience with running Qubes on old MacBooks - I've
always used rEFInd to circumvent boot issues, and everything seems fine.
That's 3.2 - I cant speak to 4.0.