To make your rifle lower a short-barreled rifle you use a Form 1. You are the manufacturer of the SBR.
For the suppressor, unless you're making it yourself, ie.: milling the baffles and misc parts with your own hand, the Form 4 is used. If you are in fact manufacturing the silencer, then use a Form 1.
either way, you will need two stamps.
ETA: If you have a registered receiver (full auto), you do not need a Form 1 to manufacture a SBR. There is no barrel length restrictions on machine guns. But you'd still end up will 2 stamps (1 for receiver, 1 for silencer).
~rj
ps: exception, if your silencer is permanently attached, and brings your barrel length to 16" then you will only need one stamp for the suppressor.