If you do a search for Dale Brown or Megafortress, there are several
drawings & photos (retouched, of course) of Dale Brown's "Old Dog" with
4 engines. It has the other Old Dog mods, but it has the 4 engines.
There's a book by Walter J Boyne about the B-52 that has some drawings
of 4 engine B-52s. One with 1 on each pylon, & another with 2 huge 'fans
paired on the inboard pylons. These are probably '70s, maybe '80s
"proposals".