The Epson PX-4 (HC-40 or HX-40) is a portable CP/M based computer introduced in 1985. The screen consisted of 40x8 characters physically, but 80x25 or 40x50 virtually, making it nearly compatible with the Epson PX-8 Geneva. It can be operated with a nickel-cadium battery pack (Epson RB 105), 4xAA batteries or a 6V 600mA DC power supply.
It was intended as the successor to the portable Epson HX-20, which was very popular for use on the road.
Another feature of the PX-4 was its high modularity. By taking the ROM capsules from the Epson PX-8 Geneva, it added a cartridge bay (similar but incompatible with the Epson HX-20), for which Epson various printers, micro-cassette drive, modem, EPROM writer, DMM (Digital Multimeter module), RAM and ROM cartridges. Third parties could make custom cartridges. The modem, EPROM writer and DMM required user programs. The system allowed BIOS extensions (user BIOS).
Other features were the serial and RS232 port, barcode reader interface like the Epson PX-8 Geneva. New were a cassette port and a parallel printer port.
The keyboard was also easy to replace, allowing for country-specific layouts, as well as custom layouts, such as the 'item keyboard' that turns the PX-4 into a cash register. This trend was continued by the Epson PX-16 for which even 'item keyboards' with touchscreens were available.
Internal RAM was 64K, part of which could be reserved as RAM disk. An external RAM disk can be plugged in, creating a 120K RAM disk, leaving the internal RAM as user BIOS and workspace.