First computer I ever used was my father's TRS-80 Model 4; Zilog Z80 processor running at 4MHz, 64kb RAM 80x24 monochrome display, and two single-sided (180kb) floppies.
It contained the Model III ROM, and would act exactly like a TRS-80 Model III if booted with a Model III OS or with no disk in the drive. This came in handy since the Model III version of Scripsit (Tandy's craptastic word processor) was cheaper than the Model 4 version. I didn't use anything but it's cassette BASIC until the late 1980's when I tracked down the TRS-DOS 6.2 disc and actually got to use the features of the Model 4.
Unfortunately, I destroyed one system (we had two) when I replaced a faulty disk drive with a double-sided one. The system was one unit (the keyboard, motherboard, and monitor were in the same case), and when I put it back together I smashed the monitor
I still have one that worked the last time I tried it, but emulating it with
xtrs is much easier these days (not as much fun though).
Don't ask me why Tandy decided to use Roman numerals for their first three systems, but not for the Model 4. There's some clues in
this post from comp.sys.tandy. I suspect heavy drug use in Tandy's marketing department
