There are requirements for both the Canadian and American governments in this case.You should be able to get the information on the Canadian Firearms Centre website.For Canadian import regulations havwe a lok at this:
Canadian Firearms Manual
A reference to American export regulations can also be found there :
Special Bulletin
You can also sometimes find pistols that have been properly modified.I picked up one with a six inch Holland & Holland barrel.
The other alternative is to have a qualified gunsmith remove the 4" barrel and purchase it with no barrel(legal to do, the barrel is simply a part,not a firearm).OK to do if the gun is a unmatching shooter anyways.