scheme :: MULTOP_1:sch 7
Lambda4D{ F₁() -> non empty set , F₂() -> non empty set , F₃() -> non empty set , F₄() -> non empty set , F₅() -> non empty set , F₆( Element of F₁(), Element of F₂(), Element of F₃(), Element of F₄()) -> Element of F₅() } :

ex f being Function of [:F₁(),F₂(),F₃(),F₄():],F₅() st
for x being Element of F₁()
for y being Element of F₂()
for z being Element of F₃()
for s being Element of F₄() holds f . [x,y,z,s] = F₆(x,y,z,s)