Some of the first microprocessors had a 4-bit word length and were developed around 1970. The TMS 1000, the world's first single-chipmicroprocessor, was a 4-bit CPU; it had a Harvard architecture, with an on-chip instruction ROM with 8-bit-wide instructions and an on-chip data RAM with 4-bit words. The first commercial microprocessor was the binary coded decimal (BCD-based) Intel 4004, developed for calculator applications in 1971; it had a 4-bit word length, but had 8-bit instructions and 12-bit addresses.
The Saturn processors, used in calculators such as the commonly used HP-48 scientific calculator, are 4-bit machines; as the Intel 4004 did, they string multiple 4-bit words together, e.g. to form a 20-bit memory address, and most of its registers are 64 bits, storing 16 4-bit digits. Its instructions were 10 bits wide. (Current HP calculators use an ARM processor to emulate a Saturn processor.)
The 1970s saw the emergence of 4-bit software applications for mass markets like pocket calculators.
While 32- and 64-bit processors are more prominent in modern consumer electronics, 4-bit CPUs continue to be used (usually as part of a microcontroller) in cost-sensitive applications which require minimal compute power. For example, one popular bicycle computer specifies that it uses a "4 bit 1-chip microcomputer". Other typical uses include coffee makers, infrared remote controls, and security alarms.
With 4 bits, it is possible to create 16 different values. All single digit hexadecimal numbers can be written with 4 bits. Binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by four bits.