Solution to 4.

 

~av{(avb)v~a}

 a  b  ~a  avb  (avb)v~a  ~av{(avb)v~a}
 T  T  F  T  T  T
 T  F  F  T  T  T
 F  T  T  T  T  T
 F  F  T  F  T  T

 

 

The statement is a tautology, as can be seen by the fact that there are only Ts in its truth column.