Intel targeted calculators (as Commodore did, BTW; Peddle's innovation wasn't the PET, it was
convincing Tramiel to let him
work on the PET at all) because that's who their clients were -- it was a business case ready-made, with guaranteed customers.
I'm not familiar with the 4004 at all, but to a casual programmer, the 8008 would not look very different from the CPU that kicked off the home computer revolution: the 8080. The instruction set is virtually identical, and all of the registers remained the same. The significant differences were that the 8008 only addressed 16KB of memory, and had an on-chip, 8-deep stack used for subroutines. Contrast this with the 8080, which possessed a 64K address space and an off-chip stack addressed by a dedicated pointer register (SP). The 8080 would go on to provide the base instruction set with which Gary Kildall would write CP/M, which itself would later be cloned into Q-DOS, purchased by Microsoft and rebranded MS-DOS, and wrapped with a GUI we all know and love to hate today as Windows.
Going back on the hardware side, Intel created a barely known descendant called the 8085, virtually identical to the 8080 as far as software is concerned, whose architecture dominated the creation of the 8086, thus giving rise to the much loved-to-be-hated "x86" architecture today. While not
binary compatible, assemblers for the 8080 can be written to emit opcodes for the 8086, and yes, that means you can still run 8080 software unchanged on Intel Pentium 4 Xeons today.
The 8008's life purpose, by the way, was not to serve inside calculators, but to serve as the heart of mainframe dumb terminals (hence the 8-bit wide byte, as the IBM S/360 and later models defined their smallest addressible units to use 8-bit bytes! So while IBM created the 8-bit byte, it was Intel who forever immortalized it).
How ironic, then, that Texas Instruments chose the Z80, itself an 8080 spin-off, after all these years, to power their hald-held graphic calculators. Seems somehow appropriate, no?
![Wink ;)](./images/smilies/icon_wink.gif)