\begin{verbatim}
architecture Behavioral of Add4Bits is

  signal bufA : STD_LOGIC;
  signal bufB : STD_LOGIC;
  signal bufC : STD_LOGIC;

  component Add1BitA is Port (
    X : in STD_LOGIC;
    Y : in STD_LOGIC;
    Ci: in STD_LOGIC;
    O : out STD_LOGIC;
    Co: out STD_LOGIC);
  end component;

  component Add1BitB is Port (
    X : in STD_LOGIC;
    Y : in STD_LOGIC;
    Ci: in STD_LOGIC;
    O : out STD_LOGIC;
    Co: out STD_LOGIC);
  end component;

begin

  first : Add1BitB port map (
    X  => A(0),
    Y  => B(0),
    Ci => C,
    O => R(0),
    Co => bufA);

  sec : Add1BitB port map (
    X  => A(1),
    Y  => B(1),
    Ci => bufA,
    O => R(1),
    Co => bufB);

  third : Add1BitA port map (
    X  => A(2),
    Y  => B(2),
    Ci => bufB,
    O => R(2),
    Co => bufC);

  fourth : Add1BitA port map (
    X  => A(3),
    Y  => B(3),
    Ci => bufC,
    O => R(3),
    Co => Rc);

end Behavioral;
\end{verbatim}