Its Form 1 if you are making the SBR yourself, and Form 4 for a transfer from another party (i.e. if you are getting the SBR through a Class 3 dealer or from another private citizen). In either case you must also get a local CLEO (Chief Law Enforcment Offficer, such as a police chief or a sheriff) to sign on the appropriate line on the Form. When you mail the form to the ATF you must also include a completed federal proof of U.S. citizenship form, two passport style photos, two completed federal finger print forms (you can take the fingerprinting forms to your local sheriff or police department to get the fingerprinting done), and of course a $200 check made out to the ATF for the NFA tax stamp. By the way, $200 is also the tax charge for an SBS, machinegun, or DD. For an AOW the charge is also $200 if you are the maker, but only $5 if you are getting it through a transfer.
If you can't find a local CLEO willing to sign the ATF form, you can instead form a LLC, corporation, or trust to have the NFA firearm transferred to. In this case since ownership of the SBR is being transferred to one of these legal entities rather than to a private citizen, there is no need for a CLEO signoff, and you also don't need the passport photos or fingerprint forms. Instead when you send in the form to the ATF, all you have to include with it is the U.S. citizenship form and the signed notarized documents showing that the LLC, corporation, or trust exists.
Of course no matter what, first check that your state has no laws barring possession of NFA firearms. If you decide to go the LLC, corporation, trust route, I also of course strongly recommend seeing a lawyer, to make sure the LLC, corporation, or trust is set up properly and in accordance with your state's laws.