After a TFW has reached their four-year cumulative duration limit, they will not be granted another work permit in Canada for an additional four years.
http://www.cic.gc.ca/english/department/media/backgrounders/2011/2011-03-24.asp
What are the exceptions to the four-year rule?
There are exceptions to the four-year rule that allow some types of foreign nationals to work in Canada beyond the limit.
A processing officer can approve a work permit application to allow foreign workers in certain situations to work more than four years in Canada, assuming all other criteria have been met.
These categories are the exceptions for which a work permit can exceed the four-year limit:
NOC O and A (including spouses/common-law partners of these skilled temporary foreign workers)
All Labour Market Impact Assessment-exempt categories (excluding spouses/common-law partners of NOC B workers)
Seasonal Agricultural Workers
Jobs that do not need a work permit
Permanent resident applicants who have received a positive selection decision or approval in principle
Provincial nominees applying for an employer-specific work permit
http://www.cic.gc.ca/english/department/media/notices/2015-04-01.asp