If you have money and the time I have already talked to EFK Dragon and they were willing to make a 9MM, 40 or .45 ACP barrel @ 107MM, then pay them for a gun its self and the barrel (they said theyd credit a bit for the factory barrel) and have it under their FFL, then get someone like Questar or do the paperwork your self and have it sent into the CFC.
4 would be a sweet gun as the 5 is just to big IMO. Tho for the $$ that would cost there are other already short guns that I'd prefer more.