Trying to generate a signal.

Tricky's answer is certainly the most elegant way to do exactly what you asked.

If you wanted to test all possibilities, another way would be to use an unsigned 8-bit signal and just increment it by one each clock cycle. first is the upper 4-bits and second is the lower 4-bits. You could even get rid of first and second and just write:

    signal counter : unsigned(7 downto 0) := (others => '0');
process
begin
    wait until CLK'EVENT and CLK = '1';
    counter <= counter + 1;
    a3 <= counter(7);
    a2 <= counter(6);
   ...
    b3 <= counter(3);
    ...
    b0 <= counter(0);
end process;

If your operands were larger, say 32-bits, you could increment by a value > 1 to limit the number of tests.

If you're allowed to make a and b arrays (maybe call them first and second), it would get even simpler and then easily scales to other size operands.