CCI SV will stay subsonic in a 16" barrel... I use them all the time on my non-SBR'd 15-22.. Never a sonic crack, even in winter.
Thus, the CCI-SV will stay subsonic in a shorter barrel as well.
The point of going to a shorter barrel is (amongst other things) having high velocity ammunition (which is often more plentiful, and less expensive) be subsonic.
Since the 15-22 is a blowback, a 4.5" barrel might be a bit too short without lightening the bolt or the spring (for example, the 15-22 pistol has a lightened bolt...).
7-10" should do well without any other modifications.
Depending on what your reasons are for going the SBR route, you might consider an Innovative Arms integral suppressor; they cut the barrel to 10", pin and weld a 16" suppressor. That means it's subsonic for even high velocity ammo, the length is the same as your current rifle, weighs about the same as your current rifle and does NOT require a form 20 to cross state lines. So, it's still one stamp (Form 4 instead of a Form 1).
The downside of the IA integral is that you can't change the handguard, and you can't reuse the suppressor on another firearm (except another 15-22...)
HTH...