Why did McDonnell Douglas build the C-17 Globemaster with 4 engines?

The C-17 Globemaster III's four-engine design holds the key to its unrivaled capabilities, but was this a choice of power or necessity?
AOL

C-17 ‘safely towed’ from runway after landing in Charleston with collapsed nose gear

NORTH CHARLESTON, S.C. (WCBD) – A C-17 Globemaster III aircraft that landed at Charleston International Airport with a collapsed nose gear has been safely towed to a hangar. The aircraft, assigned to ...