After lots of research, I bought a QuadStar II made by Bio Medical Systems.
It can drive up to 8 electrode pads.
I mainly use rechargeable nickel/metal/hydride batteries, but it also has a nice 'wall wart' AC supply with a convenient super-long cord.

I'm grateful to my Dr. who wrote a prescription so I could treat myself at home.

As a tech, I'm 'into' the electronics part of the therapy - still trying to learn more.

Used IFC (4 pad array) for my sore shoulder, knee, amd my wife's sciatica.
Running TENS in my regular foot soak for neuropathy.

Would enjoy hearing from anyone else with a QuadStar and how they use it!