Amlogic S905 and S905X support a maximum of 2GB Ram. See S905_Public_Datasheet_V1.1.4.pdf and Amlogic S905X Processor Specifications, if you do not believe me.
You either have a box with S912 (which supports up to 4GB, if I remember correctly) or your seller tricked you (which would not be the first box with some "fake" ram).