I personally would be hard pressed to trade my A0 for an A4. This one is easy. An A model mandolin is symmetrical, and teardrop-shaped. If the instrument has an adjustible bridge and a date prior to 1921, it is most likely a replacement bridge. While the bridge may even come from the Gibson factory, it still is not original to the instrument.
Plain tailpiece cover.
Shaped hardshell or canvas case. A or A0 Brown or black finish, binding only on face and in soundhole. One ring of purfling around the soundhole. Pickguard that is pinned into the fingerboard and bridge, clamped to the side of the instrument. Pearl dots on fingerboard.