I do not wish to rewrite your letter as I might have written it, so I've tried to make only those changes that I thought were necessary. These are only my suggestions and there are many other (and possibly better) ways to write this letter.
I would like to request four days' leave (of abscence), starting on 26th Dec., as I have to go back to my country (you should also give a reason for having to leave, however vague). However, there is a chance that my work might be delayed. So kindly let me know if there is any urgent work that needs to be done on those days and if it is possible to (go back to) leave for my country?
If it is not possible to take leave for the whole of the requested period because of the nature of my/the work, I (will) could leave only (for the period of time) when (I have a chance) the circumstances allow.
Student or Learner