We fly from Leeds or Manchester and aim to arrive 2-2.5 hours before take off.
Allows a little spare time in case or crash, puncture, breakdown, roadworks etc, trouble finding a parking space, bag drop off or security queues, although we tend to use the Meet and Greet parking these days after a bad experience with Jetparks at Manchester where you have to find your own space in a massive car park and get the bus in and it took over an hour to find a space, get on the bus, drop off bags and get through security.
We were a bit late anyway due to traffic on the motorway. We didn't miss the flight but we needed to get food in the airport and didn't really have time for anything other than grabbing a Boots meal deal, which was disappointing when we were looking forward to a leisurely hot meal and a pint to start the holiday.
The poster saying about Jet2 sending a too small a plane could be correct, they do have different sized planes, or have done in the past, seating 148, 189 or 235 passengers. I've certainly been on bigger and smaller planes with them.
en.wikipedia.org/wiki/Jet2.com#Fleet
4 hours before take off is a bit unnecessary unless there are particular circumstances, but for those who arrive at the last minute, insurance companies wouldn't help them in the case of missed flights like a PP mentioned, as they would expect you to leave more time than that. It depends what sort of flight you are on as to how expensive and inconvenient missing a flight would be. If you were going on holiday, you might be able to get a later flight, but it is likely to be very expensive.
If you were going on a short business trip, the event could have happened by the time you get a later flight. Sometimes there is only 1 flight a day or a few a week from regional airports, which would obviously be more of a problem than flights between major airports that have several flights a day.