As I understand it, Form 4 means the NFA item is owned by a private citizen, as opposed to a SOT (Class 3) dealer, who can transfer the item to another dealer without paying for a tax stamp.
To transfer it to another private citizen (outside of the state) would require two transfers for one item, i.e. one transfer to a dealer in the purchaser's state and another transfer to the new owner, necessitating two tax stamp purchases and 2 BATF approvals.
Inside the same state would require only one transfer - to the new owner - one tax stamp and BATF approval.
Again, I'm not positive, being a rookie myself
Go to
www.subguns.com and click on "Firearms Laws" for good info on the legalities.
Nick