Hi Thomas.
Thanks so much for uploading the handbook, good stuff !
Very hard to find these old documents.
So your 4000 has the Mark II cradle with the up/down push buttons.
My Mark I version has the older rotary controls.
So I now understand the history of this model better.
The Mark I cradle version was released in 1983, & I believe was produced for less than 1 year.
The Mark II cradle was released in early 1984 & produced for a couple of years at least.
Now, your Mark II version definitely has System A/B selection available.
But I do not know if my earlier version has this, I suspect not.
As far as I know, most if not all AMPS phones made in 1983 did not have user system selection.
Thanks again. Cheers, Chris.