A Form 3 is used to complete a tax-free transfer between two dealers, or SOT's.
A Form 4 is used to complete a tax-paid transfer between two individuals.
In the case of selling an NFA item, such as a suppressor, between two residents of different states, neither of whom is an SOT (Special Occupational Taxpayer), you would need to pay $400. The first $200 would be paid to transfer the NFA item from the owner to the SOT. Once that transfer is approved, and the SOT takes ownership, the next $200 is paid to transfer the item from the SOT to the buyer.