:-dynamic(fourth/2).
square(X,X2):-X2 is X*X.
fourth(X,X4):-square(X,X2), square(X2,X4), assert_if_new(fourth(X,X4)).

% utility predicate: If known fact - OK!, or-else add it to the data base
assert_if_new(Fact):-clause(Fact, true), ! ;
	asserta(Fact).
% notice the use of a cut(!) to make sure that both alternatives
% can not be done together by backtracking