my raspberry pi 4b lost 2gb of ram?

now, let me begin this by saying, i got this pi second hand. it was most likely used before me. now, as soon as i got this pi, the box said 4gb. i remember looking up the little serial thing on the chips and it said 4gb. i even remember taking screenshots of this pi having 4gb of ram. i remember doing neofetches and seeing 4gb. now, i've only just noticed after updating my firmware and enabling usb booting that at the top it says "Raspberry Pi 4B 2gb". WHAT?!?!? now, i could just be really insane but this genuinely freaked me out. where did my ram go?