WASHINGTON — A fourth patent at the centre of the dispute between Research In Motion Ltd., the maker of the Blackberry e-mail device, and NTP Inc. was rejected Friday in a non-final action by the U.S. Patent and Trademark Office.

The patent is one of the five at the centre of the long-running dispute between NTP and RIM. The PTO has already rejected all five NTP patents in so-called first office actions. It has now rejected four in a second round and RIM is pinning its hopes on the PTO validating its rulings in final patent rulings.

However, patent-holding company NTP won a 30-day extension in the PTO's re-examination of the patents, giving it more time to file a defence before any final rulings are issued.

Meanwhile, U.S. District Court Judge James Spencer is set to consider imposing a U.S. ban on BlackBerry sales and e-mail service at a hearing sometime after Feb. 1.

In Toronto, RIM shares closed at $76.75, down $1.76, on Friday.

On the Nasdaq Stock Market, they dropped $1.09 to close at $66.01 US.