What does 4x4 アルミ板金 mean?
4'x 4' also called 48“x 48”, because this is an imperial measure. The single quote ' refers to the foot. 4' and A' respectively indicate 4 foot and 4 foot, 1 foot=304.8 mm, 4'=1219 mm
4'x4' アルミ板金 indicates the width of アルミシートプレート × length = 4 foot × 4 foot = 1219mm × 1219mm.