Publishers.Merge4
A publisher created by applying the merge function to four upstream publishers.
Declaration
struct Merge4<A, B, C, D> where A : Publisher, B : Publisher, C : Publisher, D : Publisher, A.Failure == B.Failure, A.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output