why do freight companies tend to operate four engine aircraft rather than twins, even though they carry the same load ?
Because 747 happened to be 4 engine, and MD11 happened to be 3 engine. All other advantages are very much secondary and would never seriously influence decision making, like 3-engine ferry capability etc. Would 747 be twin engined, freight companies would tend to operate twins.
777F just entered the market so in 10-15 years this will change.