In the simplest of solutions, you could use a matrix switch to route your four USB lines to each DUT. NI has some Ethernet-based hardware that you could use to pull this off, I think.
We have had to do USB switching before, and used the J-Works SSB118. However, it doesn't fulfill your Ethernet requirement.
As for 1-to-all versus 1-to-1, I don't think the former would work because each host (your DUT) expects that only one host will be on the bus - there would probably be a mess of collisions if you plug x DUTs into one dongle.