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 different 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 Apple Macintosh specific variants of these machines with the '4M' designation. 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, as well as the spread of LaserJet 4 models from personal use up to heavy business use.

The LaserJet 4 series has now been discontinued, and Hewlett Packard themselves have recommended the HP LaserJet 4250 as a replacement for the 4/4M/4+/4M+ models. However the driver for the HP LaserJet 4 is in even wider use than the actual hardware since it can solve some tricky software problems with printing. The driver is very standard and can be used with old software with success, even when applied to other makes and models of printers.

The LaserJet 4 printer was first introduced in October 1992  as the replacement for the HP LaserJet III. The printer used a (then new) Canon EX ("LBP-EX"[4]/P-270) laser-xerographic engine, printing 8 pages per minute (PPM) at 600 dots per inch (DPI). The LaserJet 4 was the first popular laser printer to first print at 600 by 600 dots per inch. The printer supported the PCL printing language, and could be upgraded with an optional accessory to use the PostScript language.

The LaserJet 4 was the first LaserJet to use the Bitronics bi-directional parallel interface allowing communication between the host PC and the printer. This also allowed communication on the network allowing easier print network management . The LaserJet also was the first LaserJet to be shipped with TrueType fonts, with 45 scalable typefaces built-in to the printer; this allowed the font on the screen to match the print output.

The EX engine in the LaserJet 4 (and the upgraded EX+ and EX-II engines in the 4 Plus and HP LaserJet 5 respectively) was a significant improvement on the previous Canon CX and SX printing engines, and allowed Hewlett Packard to dominate the business laser printer market. The LaserJet 5 was later released as a "better" LaserJet 4 Plus.

Macintosh variants

In all the models of the four series an 'M' designation identifies an Apple Macintosh specific version, with additional accessories for network connectivity (JetDirect (Ethernet)/LocalTalk), PostScript Level 2 support and more memory, built-in as standard

4Si
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, and could be upgraded with a duplexing unit.

4L and 4P
HP LaserJet 4P
HP LaserJet 4P
The LaserJet 4 series also had additional smaller printers in the range: the personal-use 4L and the small-business use 4P (and the Macintosh specific 4ML and 4MP). These printers both used the Canon PX engine,[11] 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 4L was the first LaserJet with power-saving technology which turned off the printer when not in use .

 4+/4M+

4+/4M+

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.[14] This also provided the option for duplex printing (automatic double-sided printing) with the purchase of an accessory. The original 20MHz processor was also upgraded to 25MHz, with the addition of Hewlett Packard's Memory Enhancement Technology. The printing mechanism was also improved to allow increased tones of grayscale .

 4V
The 4V and 4MV were B-size printers for graphic artists[16] using the Canon BX-II engine. Hewlett Packard did not consider the original BX engine as "good enough" for their LaserJet series. The 4V/4MV were released in 1994 and dominated the market due to their competitive price. The 4V series was the first HP printer to offer an internal hard disk option.

4LC and 4LJ Pro
The 4LC and 4LJ Pro were printers based on the 4L platform modified for the Chinese and Japanese markets respectively. The 4LC was the "first printer designed exclusively for the Chinese market" , and was released by HP in April 1995. This was followed by the Japanese 4LJ Pro in May 1995. Both of these printers used a 2 byte PCL that had been recently developed for Asian fonts, and came with memory upgrades and Chinese/Japanese fonts. Both printers worked at 600 DPI, with 2MB RAM.