The number of distinct Boolean expressions of 4 variables is

If the functions W, X, Y and Z are as follows

W= R+$$\overline P Q + \overline R $$ S
X = $$X = PQ\overline R \,\overline S + \overline P \,\overline Q \,\overline R \,\overline S + P\overline Q \,\overline R \,\overline S $$
Y = $$RS + \overline {OR + P\overline Q + \overline {PQ} } $$
Z = $$R + S + \overline {PQ + \overline {PQR} + P\overline {QS} } $$

The circuit shown in figure converts

The circuit shown in figure has 4 boxes each described by inputs P, Q, R and outputs Y, Z with
Y = $$\,P \oplus \,Q\, \oplus \,R$$
z= $$RQ + \overline P R\, + Q\,\overline P $$ The circuit acts as a