Hewlett Packard's PX engine-based LaserJet 4L was introduced in 1992 as a low-cost replacement for the LX-based LaserJet IIP and IIIP printer models. The 4 page per minute (ppm) 4L offered several features to meet the wide range of printing needs of the Small Office Home Office (SOHO) printer market, like improved paper handling for a variety of media, increased processing speeds, improved graphics capability and a price of less than $1,000.
In 1993, HP introduced the LaserJet 4P based on the PXII engine, a variation of the PX engine. The LJ4P offered the same 4 ppm print speed as the LJ4L, but featured 600 dpi resolution. While 600 dpi resolution had been a "standard" feature of printers in the 11-15 ppm and 17-21 ppm printer segments, the LaserJet 4P was one of the first printers to offer 600 dpi to the expanding SOHO market.
The design of the PX cartridge is somewhat unusual compared to the design of previous Canon cartridges such as the SX, NX or LX. The PX hopper and waste bin sections are held together by 12 holding clips located around the perimeter of the cartridge instead of the more typical screws, pins or springs.