Why would Christmas Day and Boxing Day be taken from your Annual Leave allowance?
Aren't they Bank Holidays?
If you had 4 days holiday left to take (pro-rata from 1st April 2014 - 1st April 2015) and left mid December, is it not just a case that your holidays were readjusted pro-rata to 1st April 2014 - 14th December 2014, meaning you ended up with less than originally given for a full year?
And don't worry about being on the 'emergency tax code' - it's just the same as the standard tax code but on a month 1/week 1 basis.
Which again, if you have been earning over 10K, is not going to make much difference.
I'm thinking this is exactly what has happened.
If OP had 28 days holiday a year that is approx 2.3 days a month. He has worked 8.5 months so that gives him around 20 days holiday to use.
He thinks he has 4 unused days, so if this is taken from his annual allowance (28) that means he has taken 24.
He has only accrued 20 days, but used 24 - therefore he owes the company 4 days.