How to set value of a signal in process?

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

entity counter4 is

   port(count:out std_logic_vector(3 downto 0);

        clk:in std_logic;

        reset:in std_logic);

end counter4;

architecture behav_counter4 of counter4 is

   component ha port (a: in  std_logic;

                      b: in  std_logic;

                      sum: out std_logic;

                      c_out: out std_logic);

   end component;

   component fa port (a, b, cin : in std_logic;

                      sum, c_out : out std_logic);

   end component;

   signal ain,s,c:std_logic_vector(3 downto 0);

   signal bin:std_logic_vector(3 downto 0):="0001";

   --configuration specification   

   for all:ha use entity work.ha(rtl);

   for all:fa use entity work.fa(fa_behav);

   begin

      u1:ha port map(a => ain(0), b => bin(0), sum => s(0), c_out => c(0));

      u2:fa port map(a => ain(1), b => bin(1), sum => s(1), cin => c(0), c_out => c(1));

      u3:fa port map(a => ain(2), b => bin(2), sum => s(2), cin => c(1), c_out => c(2));

      u4:fa port map(a => ain(3), b => bin(3), sum => s(3), cin => c(2), c_out => c(3));

      counter:process(clk, reset) --process(sensitivity list)

      begin

         if reset'event and (reset = '1') then 

            ain <= (others => '0'); 

         elsif (clk'event and clk='1') then

            ain <= s; 

            count <= s;

         end if;

      end process;

end behav_counter4;