The TRS‑80 Model 4

written by Matthew Reed

The TRS‑80 Model 4, introduced on April 26, 1983, was the continuation of the TRS‑80 computer line that began with the Model I in 1977. The Model 4 was officially launched for the press on April 27 at an event sponsored by the Boston Computer Society.

The Model 4 was 100% compatible with the Model III and was able to run all Model III operating systems and applications. The designers of the Model 4 had taken great pains to ensure this complete compatibility to avoid the same kind of problems encountered when the TRS‑80 Model III was introduced. The Model III had been only partially compatible with Model I software and lack of compatibility was viewed as a costly misstep by Radio Shack. Don White, the Model 4 product line manager, stated, “I took it once on the Model I/III incompatibility and I won’t go through that again.”


The Model 4 originally was available in three configurations:

  • 16K cassette only (catalog number 26-1067) for $999.00
  • 64K with one floppy drive (catalog number 26-1068) for $1699.00
  • 64K with two floppy drives (catalog number 26-1069) for $1999.00

By 1985, Radio Shack had reduced the prices for the disk models to $1099.00 and $1299.00.

Model 4 advertisement from 80-U.S. Journal

Model 4 advertisement from the June 1983 issue of 80-U.S. Journal


The Model 4 strongly resembled the Model III that it replaced. The most noticeable difference was the color: white instead of the battleship grey that dated back to the Model I. The Model 4 was compatible with Model III software and hardware but had extra features only available when running Model 4 software. Some of those new features included:

  • a 4Mhz clock speed (double that of the Model III)
  • a full 64K of RAM plus support for another 64K of extended memory
  • an 80 by 24 screen with reverse video
  • an expanded keyboard with function keys
  • TRSDOS 6 operating system with Microsoft BASIC
  • CP/M compatibility

Although no version of the Model 4 included high-resolution graphics, it was always available as an option. Popular choices included the Model 4 High-Resolution Board from Radio Shack or the more common Model 4 Grafyx Solution from MicroLabs.

Early plans for the Model 4 had called for a detachable keyboard. The official reasons for its absence from the final product were given in a 1985 “Ask Tandy” column in 80 Micro:

The Model 4 doesn’t have a detached keyboard for two reasons. First, Model I owners wanted us to “put it all in one box and get rid of the cables.” Second, schools are big Model 4 customers and they prefer attached keyboards that can’t wander off.

The Model 4 was also originally designed to use the 16-bit Z800 microprocessor from Zilog. The Z800 was planned by Zilog to be Z80 compatible, yet capable of running new 16-bit software. That would have provided an ideal upgrade path for the Model 4’s future, but unfortunately Zilog never produced the part. Early versions of the Model 4 included a Z800 socket on the motherboard but the socket was removed in later redesigns.


Unlike the versions of TRSDOS for the Model I and III, Radio Shack contracted with an outside company to write the Model 4 operating system. Logical Systems (the authors of LDOS) created TRSDOS 6, regarded as one of the best features of the Model 4. TRSDOS 6 was a very powerful operating system, supporting advanced features such as device redirection, batch processing, and device filtering. It retained most of the commands and feel of Model I TRSDOS, yet added new features to use the improved hardware. Logical Systems also sold TRSDOS 6 under the name LS‑DOS, and it was well known under that name.


The Model 4 could run all Model III software and that feature became unexpectedly important. Internal conflicts within Radio Shack led to the cancellation of many of the Model 4 software projects in development before the Model 4 was released. Although some of those projects were later restarted, the cancellations meant there was very little Model 4 software available for months after it was introduced. The delay probably helped to create a stronger third party Model 4 software market. Even with software availability problems, the Model 4 remained Radio Shack’s best selling computer until 1986.

Other Versions

Radio Shack introduced the Model 4P, a transportable Model 4, in 1984. The Model 4P was software compatible with the desktop Model 4, but could be transported in its own carrying case. The final member of the Model I/III/4 line, the Model 4D, was released in 1985. It came standard with double sided drives and was bundled with DeskMate integrated software.

The Model 4D made its final appearance in a Radio Shack catalog in 1990, although it could still be ordered from Radio Shack for some time after that. This meant that the Model 4D outlasted almost all of the original competitors to the TRS‑80.

Categories: Computers