The Model 3 & 4 were evolutions from the Model 1. The Model 2 was targeted at a higher end business market and about the only thing it had in common with the 1,

The Model 3 & 4 were evolutions from the Model 1. The Model 2 was targeted at a higher end business market and about the only thing it had in common with the 1, 3 & 4 was the use of a Z80 processor. The upgrade path from the Model 2 would have been the Model 12, 16, 16B & 6000. The Model 16 added a 68000 processor and software could run on either the Z80 or the 68000. Tandy offered their own TRSDOS or Xenix for the Model 16. CP/M was available from 3rd parties. For a while in the early 80s the Model 16B running Xenix was the best selling unix system.