They look a lot alike on the outside, they look a fair amount alike on the inside. BUT, the 4004 series uses 4 matched sets of 4 metal can power transistors, (16 altogether) while the 8008 series uses non-matched plastic case Hitachi power transistors, mounted on a wholly different circuit board/heat sink assembly. There are 16 Hitachi units in the ST, and 24 in the BB/MkII.
First guess: The metal-can transistors will handle more current than the plastic case ones, which is why there are more of the plastic case units in the BB: It takes more to output as much as the 4004.
At any rate, the plastic case units are cheaper and more readily available. That is one of the primary reasons for the switch to the 8008 series: Mondial could no longer buy the older style power transistors in the quantities needed.
The 8008ST uses a single 2000VA double-wound transformer, the BB/Mk II uses a pair of 1100VA transformers, and double the storage capacatance of the ST. I think the 4004 is more like the BB than the ST in regard to power supply transformers and capacatance.
Far as I know, the 4004 series is also DC servo, and good to 5 hz.