pares([], []).
pares([X|Xs], L) :- X mod 2 =:= 0, pares(Xs, Laux), append([X], Laux, L).
pares([X|Xs], L) :- X mod 2 =\= 0, pares(Xs, Laux), L = Laux.

?- pares([1,2,3,4], L), write(L).