You are engraving the receiver, barrel, or suppressor tube because you are the "manufacturer" and thus required to engrave the name, city, and state. This is when you use a Form 1.
A Form 4 is for the transfer of an item that has already been manufactured and registered with the BATFE/NFA Branch. You would not engrave an item that is already manufactured and registered. For example, if you want to make an AR receiver you already own into an SBR, you fill out the Form 1 and send off with required documents and payment. When the Form 1 is received back approved, you engrave your name/trust name with city and state on the receiver in a certain font size and depth (see ATF regs). If you want to purchase a factory built and thus factory registered SBR, you would have it transferred to your Class 3 dealer on a Form 3 and then fill out a Form 4 with the required documents and payment. Since the factory registered/built SBR has the manufacturer name, city, and state on it, it does not require further engraving. It would look like any other AR you own, but would be an NFA firearm. When the Form 4 is received by your dealer approved, you go to your dealer to complete the transaction and take the SBR home.
Many folks purchasing their first suppressors buy a .30 cal that can be mounted on different weapons using the quick detach brake/flash hider mount. They also buy a rimfire suppressor. The reason for the separate rimfire suppressor is that .22LR is a very dirty caliber with a lot of powder residue and unburnt powder that builds up on the baffles/internals. It is considered wise to get a rimfire suppressor that is "user serviceable" meaning it can be taken apart to clean the internals. If you use a centerfire suppressor on a rimfire, you will fill it with residue much faster and most centerfire suppressors are not user serviceable as it's not as important of a feature as it is in rimfire suppressors.