It's every 5 years, not 4 - constitutionally. And they can drag it out to 6 if Parliament doesn't sit after year 5, unless I've missed something.
Look at the 34th Parliament - it was between the November 21, 1988 election and the October 25, 1993 election. Parliament sat from December 12, 1988 to September 8, 1993.
Of course, there's mandatory election dates now - but that's not constitutional, and can be changed by Parliament. I doubt it would happen - but not as much as I'd doubt an election this year.
Added:
Here's the reference for 5 years. Section 50 of the 1867 Constitution Act:
50 Every House of Commons shall continue for Five Years from the Day of the Return of the Writs for choosing the House (subject to be sooner dissolved by the Governor General), and no longer.
Also Section 4(1) of the 1982 Constitution Act (which extends to the provinces):
4 (1) No House of Commons and no legislative assembly shall continue for longer than five years from the date fixed for the return of the writs of a general election of its members.