maxburner,
I checked my log books and found that I flew some 75 hours as an Instructor on XP688 - initially at the School of Refresher Flying RAF Manby and then on the Refresher Flying Squadron 3 FTS RAF Leeming - between August 1971 and March 1974. The Mk4s were then phased out and flying was done on the Mk5.
The Mk4s were far and away the best of the JP aircraft.