Further to UA's points...
JP3: Call it "baseline" model. Underpowered, but once energy built up, the nicest handling e.g. for aeros. Smaller engine, flown at lower speeds, uses least fuel. I did 1:30 airborne the other day at mostly 180K-190K doing a photoshoot and used ~1500lb of fuel.
JP5: Bigger motor
Streamlined nose. Electric Windows. Pressurised = quiet inside (you canot tell how fast you are going by noise). Gentleman's touring version, handles signifcantly more poorly than JP3 due heavier, and in the RAF version I flew / JP5 I now fly, no tip tanks (reduces roll rate). The weight, higher cruise speed, bigger engine = uses much more fuel than a JP3. We could use the same 1500lb of fuel in 35min TO-Ldg in RAF 300K+ low level navs
JP4: Essentially JP3 with JP5 motor. Not as fast as JP5 since still has blunt nose. Feels more powerful on takeoff than JP5 since lighter, and no pressurisation. Not quite as nice handling as JP3 - ailerons adapted due higher max IAS (400K). Fuel between the above 2, depends on how heavy your left hand is
Were not around when I was in the RAF since they were such fun the JP3s were stored until the JP4s fatigued out!
Be aware that there is an "MPD" out there restricting some JPs to +3g/-0.25g. Pretty much a waste of time flying one with that in place
It is "removed" by some expensive engineering (wings off, lugs/bushes checked / re-worked). I believe any JP with the MPD in place does have to have the work done at some point i.e. within X hours. I would steer well clear of a JP with that MPD in place, and only offer to join after the work has been done (and paid for!) / the limits removed.
NoD