4The Greater London development plan shall be treated for the purposes of this Act as a structure plan for Greater London approved under section 4 of this Act and may be altered under section 5 of this Act accordingly ; and the Minister may direct that any area or part of an area indicated by the plan (as originally approved under section 5 of the principal Act) as an area intended for comprehensive development, redevelopment or improvement as a whole shall be treated for those purposes as an action area.