it's actually 4.05" or less that is prohibited.
I could go into a lot of depth here, and waste a whole heck of a lot of time on it, or I could go the easier route, and link you to the CFC website. The CFC is the regulatory body for firearms in Canada.
Here, you will find all the information about firearms, their classifications, and the laws reguarding them in Canada. Hope it helps you out a little.