My situation is that i had three years funding, i am in my fourth year. I dont think ill be done by october, bevause of working full time and other reasons. Is it unrealistic to ask to go part time? Thanks.

Depends upon your university and supervisor. You can check for any precedents in your particular university, discuss with your colleagues and may be come up with some solution.

You mention 'other reasons' for why you might not be done in time. I wonder if one alternative might be to seek an extension if there are extenuating circumstances that have stopped you working? If you think there might be, it's worth a shot - discuss it with your supervisor. Good luck!