HP LaserJet 4

The HP LaserJet 4 (abbreviated sometimes to LJ4 or HP4) is a group of monochrome laser printers produced in the early to mid 1990s as part of the LaserJet series by Hewlett Packard (HP). The 4 series has various models, including the standard LaserJet 4 for business use, the 4L for personal use and the 4P for small businesses. Additional models included the 4Si model, created as a heavy duty business printer, and the 4V model, a B size printer for desktop publishing and graphic artists. There are also PostScript variants of these machines with the '4M' designation, where M stands for, but is not limited to, usage with an Apple Macintosh. Hewlett Packard also released an upgraded version of the LaserJet 4/4M known as the 4 Plus ('4+')/4M Plus ('4M+'). The LaserJet 4, especially the 4/4M/4+/4M+ models, have become known for their durability, mainly due to their reliable construction, as well as the printers built in PCL (and optional PostScript) printer language support which is still used in computers to this day. Hewlett Packard dominated the laser printing sector during this time in part due to their reliability, relatively affordable pricing, and the spread of LaserJet 4 models from personal use up to heavy business use. The LaserJet 4 series was discontinued in the 1990s, and Hewlett Packard recommended the HP LaserJet 5 series as a replacement for the 4 series. However the driver for the HP LaserJet 4 exists in most, even older, software products and is a popular substitute driver for other PCL compatible printers.

The EX (and EX+) print engine is known for its reliability and longevity, and in comparison to earlier generations of laser printers, these machines have acquired a 'workhorse' reputation. The engine has a high monthly duty cycle (20,000 pages) and it is not uncommon to find units still functioning well after a million or more page impressions. However, any printer mechanism is subject to wear and degeneration, and typically printers still in use have now seen long service (the machine being discontinued with the introduction of the Laserjet 5 in 1995).

Printers that exhibit the "accordion" or "washboard" paper jam problem with the paper tightly concertinaed or folded in the exit paper path (often behind the rear access door) generally have problems with the paper exit mechanism. In many cases, the delivery rollers (upper and lower) in the upper or final stage of the paper exit are worn or unserviceable. Replacement kits for these rollers are easily obtainable and can solve the problem. An alternative is to enlarge the roller slightly by using heat shrink over the output rollers. The heat shrink extra diameter pulls the paper quicker and resolves the wear/slip issues. Another alternative is to use coarse quality abrasive paper such as those used by belt sanders to roughen the surfaces of the rollers. A quality abrasive is needed to ensure no particles are shed into the mechanism.

Introduced in 1992, this is the original LaserJet 4 with the Canon EX laser print engine. It prints eight pages per minute, and the user cannot extend the feeders.

The LaserJet 4 Plus (4+), released in 1994, was the LaserJet 4 with the improved Canon EX+ engine which increased printing speed to 12 PPM. This also provided the option for duplex printing (automatic double sided printing) with the purchase of an accessory (HP part number C3157A). The original 20 MHz processor was also upgraded to 25 MHz, with the addition of Hewlett Packard's Memory Enhancement Technology. The printing mechanism was also improved to allow increased tones of grayscale. These models also feature a 20 30W power saving (stand by) mode, which was not included in the original Laserjet 4.

The 4Si (and 4Si MX for the Macintosh) were heavy duty business printers, produced using the Canon NX engine. The 4Si worked at 17 PPM, could print on paper up to 8.5"x14", and could be upgraded with a duplexing unit and an envelope feeder. Besides the usual malfunctions common to laser printers with worn rollers or accumulated paper dust, older IIISi and 4Si printers eventually exhibit a "phantom paper jam." On longer documents, the printer will stop and report a paper jam, but with no visible reason why the paper was not feeding correctly. There are a couple of solenoids at the paper tray input mechanism that have felt pads designed to cushion them when they snap on, and the glue holding these pads in place can migrate to the surface. This delays the release of the solenoid, which causes the paper to start feeding slightly late. The error accumulates with each page printed, and eventually the paper is entering the mechanism so late that the printer believes that a jam has occurred. The tell tale symptom is a print job with a top margin that shrinks from page to page. This problem is fixed by removing the felt pads, cleaning off the adhesive (isopropyl alcohol works well), and placing some electrical tape or other material to provide a cushion for the solenoid. Unfortunately, accessing the solenoids requires disassembling most of the input mechanism.

The LaserJet 4 series had small printers in the range: the personal use 4L and the small business use 4P (and the Macintosh compatible 4ML and 4MP). These printers both used the Canon PX engine, which like the EX was new at the time of release. The 4L used the 300 DPI PX engine, with the 4P using the upgraded 600 DPI PX II engine.

The 4V and 4MV were B size printers for graphic artists using the Canon BX II engine.