The regulations for the conduct of doctoral examinations define a core period of three years in which you are expected to write your doctoral thesis and assume that the doctoral procedure (assessment and defense included) will be completed after four years. This is why your registration period is limited to four years.
Quite a few doctoral candidates, however, find themselves unable to meet the three-year-deadline, be it due to professional duties or for family reasons. Should it become apparent that you will not be able to finish the work on your thesis within the allotted time, please apply to the Committee for Doctoral Examinations to extend the working period for your doctoral thesis. If it is the first extension you are applying for you just need to state an estimate for the time you still need to finish your thesis. For any further extension you will have to submit proof of the status of your work and a supporting statement from your supervisor. Graduate School students should never forget to get in touch with their Graduate School and find out whether they need to file an application there as well.
If the Committee for Doctoral Examinations grant the requested extension you will receive a letter of extension you have to provide the Student Records and Registration Office with. Then your registration period will be amended accordingly.