Volume of water which must be added to 1L of HCL with a pH of 2 to produce a dilute solution of HCl with pH 4?

pH is -log[H]. - it is a logarithmic scale. You want to change pH bu two units. Dilution by ten times will change pH by one unit. Again dilution by ten times will again change pH by one unit. This means you need to change total volume by 10*10 = 100 times. Therefor you need to add 100 - 1 = 99 volumes of water to change pH by two units. You will need 99 L of water to bring pH from 2 to 4.