Most calculators, other than the most basic, have a button marked x^y (x superscript y) or similar. Assuming a starting balance of £4000 and a 5% increase compounded each week, to calculate your balance after y weeks enter the following:
4000 * 1.05 x^yy
If your calculator doesn't have a button to calculate x to the power y, I would recommend that you purchase a new calculator. You could alternatively use excel or an equivalent.