In1973 I was on 6FTS - we flew JP4's on the low-level part of the course. I had lost my A1 medical category, and transferred to nav. training. I flew most of the sorties with 'Diddy' Dave Marsh who had been a ground school instructor at Cranwell. Since I had around 90 hours on JP3s, he got me to fly the bloody thing as well - so my nav scores weren't quite as good as expected. The 4 was noticeably faster than the 3, though that's not saying much.