I have had informal conversations with people from back then, here is my understanding - If you read in the old IEEE hardware related journals from when the 4004 was available as a new microprocessor, you'd probably find references to 4004's being used in very early minicomputer add-ons and diagnostics panels used for computers with no easy way to access memory or to halt/test the processor. THe 4004 may not have been used too often in a stand-alone computer, but you'd probably find them in homebrew front panel add on interface panels. These front panels were relying on the mini's computing power, these were just interfaces.. What people at the time thought was - wow if this 4004 was just a little more powerful we could turn this front panel into a stand alone computer by adding a little storage RAM, a timing circuit, serial IO port for a teletype..