http://www.cic.gc.ca/english/department/media/backgrounders/2011/2011-03-24.asp
TFWs are asked to keep thorough records to clearly document the time they have spent working in Canada. The worker must be able to provide to the CIC or CBSA officer documents that prove there were breaks from work during the time period covered by their previous work permit. Examples of legitimate breaks from work would include:
Extended unpaid leave.
Parental leave.
Periods of unemployment.
If the officer is satisfied, the cumulative duration calculation would be adjusted.
But also note there are a number of exemptions from the 4 year rule, for example, if your work experience is at an NOC level O or A.