Why is there 4 inches of metal at on each side of this column?
The metal that you are detecting is probably the metal studs that are in each corner of the column. From the picture it looks like that column is also sheathed in drywall and so there may very well be a metal corner strip along each vertical edge as well. The metal studs width are typically going to be 3.5 inches. The drywall thickness from the adjoining sides is 0.5 inches and with the metal corner piece it adds up to the 4 inches that you are seeing.

As mentioned in comments this column is most likely a place where some part of the building infrastructure is located. It could be a structural beam, vertical routing space for water or gas piping or electrical wiring. 

As to whether you can use conventional drywall anchors to mount light weight things to the living space side it would be best to be certain what is inside this thing before you go drilling holes in it. It could be OK but then again it could be something to be highly discouraged for a variety of reasons.