KKadakia,
Your code should be like this for a pure combinatorical mux :
pMux : process (A,B,C,D,Sel)
begin
case Sel is
when "00" => out <= A;
when "01" => out <= B;
when "10" => out <= C;
when others => out <= D;
end case;
end process pMux;
A,B,C,D and out can be either what but should be of the same
type.
regards
jeandelfrigo