Wasn't the rule that sectors longer tha 60 minutes from an alternative airport require more than 2 engines? Hence 4 engined transports.
AFAIK the rule still exists, but an exception is accepted for twin jet engines with proven reliabilty of more then so and so hours per engine inop, including essential systems. That exception is certified as ETOPS, and if demonstrated at first delivery of the type as Early-ETOPS.
What is WHBM? There is no google for it.