Model III, Omikron (also a Model I mod), Holmes Engineering, and Memory Merchant. Options were available for upgrading the CRT to the CP/M professional standard of 80 columns and 24 rows, as well as eight inch floppy drives. A number of third-party manufacturers specialized in upgrading Model IIIs with high performance hardware and software, and remarketing them under their own labels. The improvements typically included internal hard disk drives, greater capacity floppy drives, 4 MHz Z80 speedup kits, professional grade green or amber CRT video displays, better DOS software (typically DOSPlus by Micro Systems Software or LDOS by Logical Systems) including the all-important hard drive backup utilities, and custom menu-driven shell interfaces which insulated non-expert users (business employees) from the DOS command line. These were touted as high productivity turnkey systems for small businesses at less cost than competing business systems from higher-end providers such as IBM and DEC, as well as Radio Shack's own TRS-80 Model II. Model 4 The successor to the Model III is the TRS-80 Model 4 released in April 1983. It has faster Z80A CPU, a larger video display 80 columns x 24 rows with reverse video, bigger keyboard, internal speaker, and its 64 KB of RAM can be upgraded to 128 KB of bank-switched RAM. The display can be upgraded with a high resolution graphics card yielding 640240 pixels. The Model 4 is fully compatible with Model III and CP/M application software. A diskless Model 4 (with 16 KB RAM and Level II BASIC) cost , with 64 KB RAM and one single-sided 180K disk drive , and two drives with RS-232C ; an upgrade for Model III owners cost and provided a new motherboard and keyboard. Tandy sold 71,000 in 1984. The Model 4 includes all of the Model III's hardware, port assignments, and operating modes, making it 100% compatible. Model III programs running on a Model 4 can access the Model 4's added hardware features (like 4 MHz clock rate, bigger video screen and keyboard, banked RAM above 64 KB). There were aftermarket software packages that made this ability available to non-programmer users. The Model 4P is a transportable version introduced in September 1983 and discontinued in early 1985. It is functionally the same as the dual-drive desktop model but lacks the card edge connector for two outboard diskette drives and for cassette tape interface. It has a slot for an internal