I'm not sure, My scenario is gonna run all the way up to 1986, so while jet bombers will replace prop engines bombers before the need for the six engine bomber arives, Not all civs are going to be able to build jets because of resource restrictions.
Jet planes need special high quality metals in their engine parts, the magnesium parts in the ME262 for instance wore out after just a few flights. I'm not sure however just how rare magnesium is.. Aluminumn would be required also for the body of the plane especialy in supersonic variants.
So I may produce a six engine bomber for civs unable to build jet bombers, but that depends on how my research on magnesium progresses.