Your barrel has to be 4.25" or greater or it will be prohibited!!
Went through that with my built. I almost had a panic when I was on the phone registering it.
So, NO, you cannot have a 4" barrel.
Please reference the section of the firearms act that prohibits a
rifle from having a barrel length under 105mm.
According to the Criminal Code, a prohibited firearm is:
a
handgun that
has a barrel equal to or less than 105 mm in length, or is designed or adapted to discharge a 25 or 32 calibre cartridge, but does not include any such handgun that is prescribed, where the handgun is for use in international sporting competitions governed by the rules of the International Shooting Union,
a firearm that is adapted from a rifle or shotgun, whether by sawing, cutting or any other alteration, and that, as so adapted,is less than 660 mm in length, or is 660 mm or greater in length and has a barrel less than 457 mm in length,
an automatic firearm, whether or not it has been altered to discharge only one projectile with one pressure of the trigger, orany firearm that is prescribed to be a prohibited firearm
The Skorpion is available in 32 acp (same section of the act) because it is classified as a rifle. As long as you don't chop the barrel, a 4 inch AR rifle is good to go, as would be a Skorpion in 25 acp, or an AR rifle in 25 acp.