The K4 is a monster! Like most have already mentioned its at its best when you fly it very aggressively using e-fighting techniques. You can afford to turn hard and scrub off speed as it will accelerate you like a bullet out of gun back to safe speeds. Flying it timid all like is a waste of time. Timid means you are allowing your opponent time to rebuild E and you will be essentially BnZing. K4 is not at its best BnZing due to the tater gun being hard to use. Keep the enemy slow and accerating slower than you and you will build a net E advantage which you can use to convert into a kill by using the vert. Its THE e-fighter par-excellence.
Make sure you get into the habit of working that throttle like mad. Dont be afraid to wind it back to zero frequently to keep you in the sweet-spot speed range. Too fast and you will suffer from a very unresponsive elevator. Keep all that power well under control and you wont have to fight the aircraft to get it going where you want it. Going over the top of loops it pays to chop throttle and pop a notch of flaps which really helps get the nose back down and that tater gun pointing at the baddy. Keep throttle well under control in the downward portion of loops etc or dives. Excessive speed is your worst enemy in the K4 and the engine will give you that speed very quickly whether you want it or not.
Keep the elevator trim in a handy position to save your lawn-dartin arnold if you get out of control too
The K4 will turn very well when you keep it in its range and will keep up with anything if you are careful controlling its speed.
Another bonus is its small - means hard to hit too
If it goes pear-shaped, then you can escape from nearly every aircraft in the set if you have WEP left and are not on the deck - WEP all but never runs out too
As others have mentioned, killing with the tatergun is art form all to itself. One which will take time to get used to, let alone master. Try firing only the tatergun. Forget the MGs, the only confuse you as their ROF and tradjectory is too different. If your hitting with MGs, you're missing with the taters. Fire only taters and only in 1/2 second bursts - eg for one audible 'thump'. You only got 60 odd taters, so use them sparingly, and use em when the bogey fills up your windscreen and you can't miss
One hit is all thats needed 90% of the time. If you see a tater hit, its all over. no confusion!
As for the P40, treat is a fun thing to do every once and a while. It is outclassed in the LW MA's. Fun it is, a competitive bird against LW monsters, it aint. Theres something really insulting about being shot down by a P40