OK, you started 26 April and the holiday year runs to 31 December, that's 250 days.
Statutory minimum entitlement is 5.6 weeks which is 28 days pro rata, a contract of '20 days plus bank holidays' is not exactly clear when calculating a part-year: as it happens 6 bank holidays fall in the period 26 April 2011 - 31 December 2011, so your actual holiday entitlement could be anywhere between 28 days / 365 x 250 = 19.18 (the statutory minimum) and and 20 / 365 x 250 + 6 = 19.70 days, in each case pro-rata of course.
First using the lower figure:
26 April to 21 August is 118 days so 118 (days employed) / 365 (days in year) x 5.6 (weeks) x 2 (days per week) = 3.62 days
22 August to 31 December is 132 days so 132 (days employed) / 365 (days in year) x 5.6 (weeks) x 5 (days per week) = 10.13 days
Total entitlement 13.75 days, you have taken 4 so that leaves 9.75 - you can't round down so that means 9 days plus AT LEAST 3/4 of a day: most people would round up to 10 days.
With the higher figure:
26 April to 21 August is 118 days so 118 (days employed) / 365 (days in year) x 20 (days) + 3 (bank holidays in the period) = 9.47 days, prorata for 2 / 5 days = 3.79 days
22 August to 31 December is 132 days so 132 (days employed) / 365 (days in year) x 20 (days) + 3 (bank holidays in the period) = 10.23 days
Total entitlement 14.12 days, you have taken 4 so that leaves 10.12 - you can't round down so that means 10 days plus AT LEAST 0.12 of a day. I'd suggest that you settle on 10 days.
You say you have taken 4 days - what happened on 29 August which was a bank holiday?
I'll assume you had that as well so you have taken 5 days and should now be given Christmas Day, Boxing Day and 3 other days (or 2 3/4 if you want to accept the bare statutory minimum).
HTH 