Artikel 4

1. The principle of the freedom of transit is set forth in article 1 of the Constitution. It shall carry with it the obligation for each member country to ensure that its designated operators forward, always by the quickest routes and the most secure means which they use for their own items, closed mails and à découvert letter-post items which are passed to them by another designated operator. This principle shall also apply to missent items and misrouted mails.
2. Member countries which do not participate in the exchange of postal items containing infectious substances or radioactive substances shall have the option of not admitting these items in transit à découvert through their territory. The same shall also apply to printed papers, periodicals, magazines, small packets and M bags the content of which does not satisfy the legal requirements governing the conditions of their publication or circulation in the country crossed.
3. Freedom of transit for parcels shall be guaranteed throughout the territory of the Union.
4. If a member country fails to observe the provisions regarding freedom of transit, other member countries may discontinue their provision of postal services with that member country.