Hi Nathan;
I know very little about the plaus 4, but I did a bit of searching and I found some stuff that might be usefull to you.
At the bottom I popped in a couple of links, with one being a photo of the back end of the plus 4:
Plus/4
COMMODORE PLUS 4.
This 1984 COMMODORE 8 bit machine is a mixture of problems and progress. Problems because of the lack of compatibility with everything else COMMODORE has made, so, own busses, own joysticks, own datasets. Problems because of the incompatibility with the C16 or C64 software! The Plus 4 had a bit of software of its own in ROM, such as a word processor, a kind of spreadsheet program and some "graphics" utility, which could be reached by pressing F1 or entering SYS 1525.
But also something better: BASIC 3.5 in stead of BASIC 2.0, which was quite an improvement!
CPU: MOS 7501 at 1.76 Mhz. RAM: 64 kB., ROM: 32 kB. for BASIC 3.5 and another 32 kB. for the software mentioned above. OS: own. Text mode: 40x25. Graphical: 320x200 monochrome or 160x200 in 4 colors. No internal drive. 1541, 1570, 1571 and 1581 could be used. I/O: 1x IEC (serial), 1 module, 1x audio/video, 1x HF, 1x datasette, 2x joystick.
ROM 22 KB built-in Wordprocessor, Database and Spreadsheet
RAM 64 KB
CPU MOS 6510
Frequency 1 MHz
OS Basic v3.5
http://obsoletecomputermuseum.org/plus4/
http://obsoletecomputermuseum.org/plus4/plus4_2.jpg