Foreign workers in Canada could only work up to four years unless their type of work permit qualified for exemption. After the completion of the four-year work period, there was no option to extend their work permits and they would have to leave Canada for another four years. As of December 13, 2016, the 4-year rule no longer applies to temporary foreign workers.
If you wish to stay in Canada on a work permit and you have already worked in Canada for four years, you are now allowed to apply for an extension of your work permit, provided that you meet all applicable requirements. If you already left Canada because you had previously met the four-year limit, you may apply for a new work permit without having to wait for four years outside Canada.