From their website at
http://www.k4usd.org
:
The K4USD Network is operated by the US Digital Amateur Repeater Club (K4USD) using multiple c‑Bridges™ to interconnect Motorola Mototrbo™ repeaters together and peering with other DMR bridge providers including DMR-MARC, DMRX, DMR+, Brandmeister, and 27 other regional DMR networks.
If you look at that page, they have repeaters connected to DMR-MARC, Brandmeister, DMR+, just a hodgepodge of networks. If I may venture an opinion, the K4USD network isn't a network within itself, but repeaters connected to other networks. Thus, you would want to connect to the networks these repeaters are on (Brandmeister, for example) and you'd be able to communicate to the K4USD repeaters that are connected to Brandmeister.
That said, nowadays, there is a lot of interconnect between the various networks; Brandmeister to DMR-MARC, for example, so it's possible to get out on some networks just by correctly connecting to others. There's too much you can do to only have one answer to your question, honestly. I suggest research and experimentation.
In a nutshell, it seems most the servers are on Brandmeister or DMR-MARC. That suggests you should be looking at the Homebrew/MMDVM connector (Brandmeister) and the DMRPlus dongle connector (DMR-MARC). Choose the server nearest you in either case.
Finally, PLEASE read the manual. Instructions for each connector are there:
http://manuals.sharkrf.com/openspot2/en/