On this page (https://wiki.linuxaudio.org/wiki/hardware_support
) the Tascam
US-4x4 is listed as "*supported* There is a Windows and Mac configuration
program, but it doesn't control much."
I own one, and recently discovered that one of the things that is quite
important if you need four distinct outs is controlling the output routing
of the line outputs; without the Windows util, basically all the signals
coming in from the PC are mixed together, along with inputs from the
preamps (based on the Monitor Balance knob), and then the output is sent as
stereo pairs out both of the pairs of line outputs.
So, I kinda reverse engineered their USB protocol a bit, and wrote a tool
to let you configure the outputs.
You can find it here: https://github.com/guysherman/tascam-util
Anyway, maybe that will help someone else out. Also, if anyone who
maintains the Wiki can let me know how to update that hardware support
page, I can link this util from there.