And don't get me wrong, you shouldn't expect to win every engagement against planes such as the K4. However, the fight is in no way as one sided as you seem to believe if you truly know the Pony inside and out and mix in some creativity and unexpectedness.
Oh.. that is very true about unexpectedness. The ponies that get me are the ones that surprise me - I see him coming in with lots of speed as usual, I initiate a lazy turn saving E to blast the pony as he goes by, reverse expecting to see him spill ahead but instead he is there still behind, flaps out and barrel rolling with me... now there an "oh ****!" moment for you. Sustained turns, radius, charts... the 51 will get me before long before they become applicable.
Also, if anyone really knows "the thing the Pony does really well in a dogfight", something akin to the Hog's scissors or the P-38's vertical performance, mayhaps I can bait them into spilling the beans by insulting our favorite plane.
Many on many engagements do not work out the same as a duel. In a furball, especially against coordinated wingmen, the best dueling moves of the 109K will get it killed fast. Climbing spirals, vertical stalls, slow sustained turns etc are likely to get you killed no matter what you fly, so your plane may as well not reply on them. You do not need a single feature of "the thing the Pony does really well in a dogfight", it is about the whole package. There are no trump cards in a furball, but generally the faster planes and those with better SA prevail and the P51D is good at those.
Good, no one said it was the best, quite the opposite.