A chip test room in Santa Clara in California late at night, on a bench a test board of green fibreglass with a single low socket, in it a small chip two centimetres long of white ceramic marbled with pale blue-grey, a matt gold lid in its middle, sixteen short gold legs, plain smooth and unmarked, a flat grey cable to a tall grey test rack with a column of small glowing red lamps, a coil of yellow paper tape with rows of small round holes, a hand in a white shirt sleeve holding fine tweezers at the chip, a single desk lamp with a warm pool of light, the window behind black.
What it was used for
The 4004 was the arithmetic and control unit of the MCS-4 set and was built to order for Busicom's 141-PF printing calculator: enquiry April 1969, the four-chip set finished in early 1971 (Intel). Per IEEE Spectrum the family comprised the 4001 with 256 bytes of program store, the 4002 with 40 bytes of data store, the 4003 shift register for input and output, and the 4004 as CPU. The Computer History Museum gives 2,300 transistors in a low-cost 16-pin package; the 4004 restoration project gives a 740 kHz clock. According to the IPSJ museum, Intel bought the selling rights back and began open sales in November 1971.
What was new
What it could do: the instruction decoding, arithmetic unit and program counter of a 4-bit computer sat on one die that took its behaviour from an external ROM instead of being wired for one task. The 4004 was not alone: alongside it the Computer History Museum lists Lee Boysel's AL-1 at Four-Phase (1969, an 8-bit CPU slice expandable to 32 bits) and Garrett AiResearch's MP944 (1970, 6 chips implementing the F-14A central air data computer). Ray Holt's paper on it was accepted for publication in 1971, withdrawn for security reasons and released only in 1998. Who counts as first therefore turns on whether one demands the whole CPU on one chip or also counts CPU slices and chip sets.
What the sources leave open: The transistor count varies: 2,300 at IEEE Spectrum and the Computer History Museum, 2,200 at EEJournal (there in comparison with the 8008). Both describe the same part and the difference is unresolved; the text uses 2,300. None of the checked pages gives a price for the 4004.