As Ursula62 has indicated, it's not the number of hours per week that you work but the number of days.
If you work a four-day week (i.e. 10 hour shifts) then you'll be entitled to 4 x 4 = 16 days holiday.
If you work a five-day week (i.e. 8 hour shifts), your minimum holiday entitlement is 4 x 5 = 20 days holiday.
If your working hours are spread over 6 days then you'll be entitled to 4 x 6 = 24 days holiday.
Your employer can determine the dates on which your holidays are taken. If, for example, in 2006 your firm is closed on the dates below, then you will only have 9 days left to be agreed between you and your employer (assuming that you work 5 days per week, Monday to Friday):
Mon 2/1, Fri 14/4, Mon 17/4, Mon 1/5, Mon 29/5, Mon 28/8 + all of Christmas week.
Chris