Art--
1. Form 4 is what you as an individual posses the gun on--a Tax paid transfer.
2. A Form 3 is a Tax Free transfer from a Dealer to a Dealer (SOT to SOT--they pay a "Special Occupational Tax" of $500 a year that covers all their transfers).
3. A Form 3 is what you want to buy a gun that is out of state on--less hassle, as it is already in a dealer's possesion, and a quicker (generally around a month) transfer to your dealer. A Form 4 is OK for an in-state purchase, as you can just pay one tax to move it off of the Seller's Form 4 onto a Form 4 for you. If the gun is on a Form 4 out of state, it has to have ANOTHER $200 tax paid to get it to a dealer (it can go direct to your dealer) where it goes on a Form 3, and then onto another Form 4 to you--that is why you will occasionally see a firearm listed as "$xxx, on a Form 4, buyer to pay taxes" (means add $200 to the cost of the gun, in addition to the tax you will have to pay to get it on a Form 4 to you.
The Supressor is also an NFA item, so you will have to pay a tax on it also when you get it transfered to you. Again, if it is on a Form 3, there is no tax to get it from the dealer who has it to your dealer. Supressors are generally cheap enough that it is difficult to sell one on a Form 4, unless you sell it to someone in your state directly to another Form 4 (i.e. no-one wants to pay an extra $200 on a $400 supressor, it's cheaper and easier to find one already on a Form 3).
E-mail if you want more info.
AFARR