FAR 25.123(c) contains en route flightpath requirements for three or four engined planes for a two engine inoperative case.
FAR 25.121 is the one engine inoperative climb requirements section; although (obviously) it only addresses OEI cases, there is implied margin for four-engined aircraft in that it requires an additional flightpath angle of approximately 0.5% for four engined aircraft after engine failure, compared to a twin after losing one engine.