My family had one of these long ago. Some years after my military service ended, I asked about it and found it had been given to my brothers in-laws and had been subsequently thrown away. I remember waiting forever for the word processor to load from the tape drive because we couldn''t afford the floppy disk drives. Radio Shack was really good with everything about these computers and I bought a book that showed me how to convert Apply programs to TRS-80. I spent hours and hours making the fireworks program. In re-living my past, I''ve been considering finding an old TRS-80 model 4 and installing all new components in it, mixing the new with the old. That way I could have a functional antique computer. However, my imagination might just be too ambitious.

I found one of these in my Grandfathers attic after he passed away. It powers on and still has the original cardboard slips in the floppies. If anyone is interested in it email me at paulvassiliou@live.com

Your description and photo of the TRS-80 Model 4 is very interesting:
First, your photo. You show a green-screen computer with split-arrow keyboard. Perhaps I should explain:
The Model 4 was produced in two distinct versions (not mentioned in your description). Initially, in April 1983, was the so-called "Non-gate array". This had a black-and-white CRT with a keyboard that had the four arrow keys split between the left- and right-hand sides of the keyboard. The up/down keys on the left, and the right/left keys on the right. This model is also distinct in that it used PAL chips for its digital logic on the PCB. Also, to include the optional peripherals (disk, RS-232), it required the installation of multiple circuit boards within the cabinet. What''s more, to expand to the maximum 128K of RAM, a special expansion PAL chip was required.
Then came the so-called "gate-array" Model 4 sometime in mid-1984. This model included ALL digital circuitry on one circuit board (including FDC and RS-232) as standard feature. It could be expanded to 128K RAM with no special PAL chip $ only the new RAM need be installed and a shorting block (jumper) moved. The gate-array Model 4 also included a green CRT and a new keyboard which had all four arrow keys grouped in a cluster at the lower-right of the keyboard. Yet another useful feature of the gate-array model was that all connectors were repositioned so that they faced directly out the back of the cabinet, rather than straight downward, as they had on the non-gate-array. (the later Model 4D also had a backspace key, double-sided drives, and included DeskMate integrated application software. In its last years the 4D also came from Tandy with LS-DOS 6.3 operating system with many enhanced features).
$-The reason your Model 4 photo is so interesting is because you show a Model 4 with a new green screen and the old split-arrow-key keyboard. Perhaps this is a gate-array computer with an old-style keyboard replacement. Then again, when Tandy was transitioning between non-gate-array and gate-array production, a very few computers were shipped with the new CRT and old keyboard because of vendor supply issues. If this is so, you have a very rare specimen on your hands! $-
Right from the beginning, all Model 4s provided for the optional high-resolution graphics board which endowed the machine with 640 x 240 resolution and 32K of additional port-accessed memory.
There were many many aftermarket upgrades available from third-party vendors and enthusiastic hackers. The clock rate of the non-gate array could be upped to 6 megahertz, and the gate array could go as far as 7 Mhz. By substituting 256K DRAMs, memory could go up to one megabyte by double-stacking the chips in the Model 4 sockets. Fortunately, the authors of the LS-DOS (TRSDOS 6) operating system made provision in the software for such memory expansion with their ingenious @BANK supervisor call. The expansion memory beyond the Z-80''s 64K limit was accessible as a RAMDISK, or at the same time (user-definable) as distinct 32K banks to applications software (such as PRO-WAM, SuperLog, Said, Allwrite). Later, a company called Hi-Tech made available their XLR8er expansion board which simply plugged into the Z-80 socket, making an easy expansion of 256K and 6.144 Mhz Z-180 (8 Mhz Z-80 equivilant). The XLR8er, since it only used the Z-80 socket, could co-exist with the other megamemory mods that only used the DRAM sockets, and the operating software was easily adaptable because its complete assembler source code was available to the dedicated hacker with publication of LSI''s THE SOURCE.
In this regard of openness of the machine architecture, I would be remiss not to mention that Tandy alone of almost all computer manufacturers, always made available to the public all of the technical data on its products. For each and every of its computers, anyone could order both the Technical Reference Manual (including schematics and theory of operation), and the Service Manuals (including maintenance and troubleshooting data).
Indeed, availability of these data made possible the many many hacker upgrades published for the TRS-80 Model 4 in its many incarnations and thus contributed greatly to its beloved and long-lived status among vintage computer enthusiasts.