?- binaryHex('11111111',X).
X = [f, f].

?- binaryHex(X,[f,f]).
ERROR: atom_codes/2: Arguments are not sufficiently instantiated

hex('0000',0).
hex('0001',1).
%etc
hex('1111',f).

binaryHex(X,Y) :-
    atom_codes(X,A),
    binaryList(A,B),
    binaryHex_(B,Y).

binaryList([],[]).

binaryList([A,B,C,D|Ds],[Y|Ys]) :-
    atom_codes(Y,[A,B,C,D]),
    binaryList(Ds,Ys).

binaryHex_([],[]).

binaryHex_([B|Bs],[H|Hs]) :-
    hex(B,H),
    binaryHex_(Bs,Hs).