I have the following function:

r12ExpUnit[n1_, n2_, n3_, n4_] := r12ExpUnit[n3, n4, n1, n2] = r12ExpUnit[n1, n4, n3, n2] = r12ExpUnit[n3, n2, n1, n4] = ((some function)) 

Here n1, n2, n3, n4 are indices in which n1 and n3 are interchangeable, and n2 and n4 are interchangeable. Is there a way to use SetAttributes to declare these two pairs of indices are orderless without declaring all four orderless?