Mine has four holes like yours but in a row.
Attachment:
Parcel Shelf LHS under mounting holes.jpg
Attachment:
Parcel Shelf LHS mounting holes.jpg
Attachment:
Parcel Shelf LHS mounting holes v2.jpg
My best guess to date is a surface mounted socket for the ancillaries (spotlight, hand-held POLICE sign, blue flashing beacon), although there's no corresponding depression in the vinyl. Three different connectors have been mentioned by ex-Police as having been used at various times - Small Bayonet Cap (SBC), 2 pin 32v and cigarette lighter.
Attachment:
Police Parts 2.jpg
Attachment:
Police accessory socket.jpg