Part of the reason is the certification climb criteria:
2-engine transport aircraft, OEI (one engine inop) - min. climb gradient = 3.0%
3-engines, OEI, min. climb gradient = 2.7%
4-engines, OEI, min. climb gradient = 2.4%
I think you've got it backwards - 2.4% for 2-engines, 2.7% for 3-engines, and I'm not sure if it's 3.0% for 4-engines
