Error

I have some problems when I try to compile mymux. This is my code:

LIBRARY ieee ;

USE ieee.std_logic_1164.all ;

Entity mymux21 is

port (A,B: in std_logic_vector (3 downto 0);

S: in std_logic;

F: out std_logic_vector (3 downto 0));

end mymux21;

Architecture behave of mymux21 is

begin

F <= A when (S = '0') else B;

end behave;

Architecture struct of mymux21 is

component mymux21 is

port (A,B,S: in std_logic;

F: out std_logic);

end component;

begin

Copy1: mymux21 port map (A => A(0), B => B(0), S => S, F => F(0));

Copy2: mymux21 port map (A => A(1), B => B(1), S => S, F => F(1));

Copy3: mymux21 port map (A => A(2), B => B(2), S => S, F => F(2));

Copy4: mymux21 port map (A => A(3), B => B(3), S => S, F => F(3));

end struct;

and the error is: "project too complex: hierarchy path is too long"