Fourth report from the Commission to the Council and the European Parliament on certain third countries' maintenance of visa requirements in breach of the principle of reciprocity in accordance with Article 1(5) of Council Regulation (EC) No 539/2001 listing the third countries whose nationals must be in possession of visas when crossing the external borders and those whose nationals are exempt from that requirement, as amended by Regulation (EC) No 851/2005 as regards the reciprocity mechanism