The Free On-line Dictionary of Computing (30 December 2018):

Intel 4004

    The world's first microprocessor, released in
   1971.  The 4004 contained 2300 transistors (compared with 5.5
   million in the 1996 Pentium Pro) and was intended for use in
   a calculator.  It processed data in 4 bits, but its
   instructions were 8 bits long.  Program and Data memory were
   separate, it had 1 kilobyte of data memory and a 12-bit PC
   for 4K of program memory (in the form of a 4 level stack,
   used for CALL and RET instructions).  There were also sixteen
   4-bit (or eight 8-bit) general purpose registers.  The 4004
   had 46 instructions.

   (1997-03-30)