When I did mine, the fees were £3,000 a year.. however that was pre-top up fees and as my BSc cost £1K a year as opposed to £3k a year ... I don't know if the PhD has gone up as well.
4 years part time? I'd say that was rather optimistic and aim for 5.. but that's based purely on personal observations and not on your research skill or writing ability which, of course, I don't know...