VHDL Code for Multiplxer using with-select statement

Below written VHDL code is for 4x1 multiplexer using with-select statement.

LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;

ENTITY mux4 IS
  PORT ( i0, i1, i2, i3, a, b : IN std_logic;
                            q : OUT std_logic);
END mux4;

ARCHITECTURE mux4 OF mux4 IS
  SIGNAL sel: INTEGER;
BEGIN
  WITH sel SELECT
    q <= i0 AFTER 10 ns WHEN 0,
         i1 AFTER 10 ns WHEN 1,
         i2 AFTER 10 ns WHEN 2,
         i3 AFTER 10 ns WHEN 3,
         'X' AFTER 10 ns WHEN OTHERS;

  sel <= 0 WHEN a = '0' AND b = '0' ELSE
         1 WHEN a = '1' AND b = '0' ELSE
         2 WHEN a = '0' AND b = '1' ELSE
         3 WHEN a = '1' AND b = '1' ELSE
         4 ;
END mux4;