To be certified, an aircraft needs to have enough power to continue flight safely if one engine fails just after takeoff.
Say a given aircraft needs 100 units of thrust to fulfil this.
If you equip it with 2 engines, they need to yield 200 units of thrust total.
If you equip it with 4 engines, they only need to yeld 133 units of thrust. So you have some savings here. People wiser than me will elaborate on weight and fuel burn implications :-)
Also, 2 engined aircraft have some limits imposed upon them for flight over water, they need to stay withing X minutes of flight to a suitable airport. Think the regulations are called ETOPS.