Although, the standard ICAO phraseology should be 'descend to altitude 4,000 feet' occasionally, some controllers may say this in order to prevent pilots from badgering them. Also, on occasion, i myself may say, 'climb to maintain FL...' when there is traffic above/ below, and quite often to non-British pilots, who for the reasons Catwalk Dweller mentions above, will start to reach a clearance limit and will ask for further climb/ descent. I guess really though, things like this are just done at the discretion of a controller.