Handguns with at least 4-inch barrels are considered "restricted" firearms and are allowed in Canada...
The limit is actually 105mm or 4.13".
This may seem like I'm nitpicking, but IMHO it's very important to recognize the critical 0.13" because of the sheer number of service pistols and revolvers in circulation in the USA with barrels that measure almost exactly 4". These guns won't pass legal muster in Canada.
That said, many so-called 4" handguns sold in the USA in the past few decades have barrels that actually measure about 4.2". This is often done specifically so that the gun can be sold commercially in Canada without the need to produce a special export version. Check the gunmaker's specs closely or get out the calipers before you jump to conclusions.