Check by ÒReverse EngineeringÓ
(Try all combinations; one combination is shown)
u
v
w
x
f1 (u, v, w, x) =
   u' v' w x'
+ u' v w' x
+ u v' w' x'
f2 (u, v, w, x) =
   u' v' w' x
+ u' v w ' x'
+ u v' w x'
1¨01
2¨10
0¨00
0
0
0
0
0
0
0
0