Why B-52 was not re-engined as a more efficient 4-engine?
There were several proposals to do just that, none of them ever went beyond the proposal phase.

Part of the reason is almost certainly the large stockpile of spare engines the USAF had (and maybe still has), taken in part from re-engined KC-135s.

Part of the reason no doubt is also that there have been near constant plans to retire the B-52 "soon" and a major project like this was never deemed economical.