for y4 being object
for X1, X2, X3, X4 being non empty set
for x being Element of [:X1,X2,X3,X4:] st ( for xx1 being Element of X1
for xx2 being Element of X2
for xx3 being Element of X3
for xx4 being Element of X4 st x = [xx1,xx2,xx3,xx4] holds
y4 = xx4 ) holds
y4 = x `4_4