Read both threads, and I'm sorry to say Ethan, the other people - harsh, as their answers may be - are right.
"It is your situational awareness that is in question" - first thing which comes up my mind for me too, when I read your questions.
Regarding "Zoom up from 4k below"..
This, is clearly an exaggeration. I'm sure you don't mean 4000 feet - enemy distance is in yards. So, zooming up 4k yards(12,000ft) to catch an enemy plane is out of the question(unless it is a rocket plane like the Me163B).
From your account, what I imagine is that you find a low furball, make a few passes, extend away at high speed and find someone behind you. You see them, and since you're at high speed, you think you can out-zoom him - but he follows you through and shoots you down.
That, is
EXACTLY what some of the base-defense expert pilots have precisely tried to master - just waiting the chance to pounce someone who thinks he's all clear. I know I try to do a lot of that, too.
No matter how fast you are, there are always many ways to reduce the distance for the attacker - if the hunter is smart, he'll use a few clever vectored rolls + power dive down to go the 'short cut', and time his drop down behind you so that he lands exactly about 300 yards behind you - even if he is much slower than you, he still has a great firing chance, because he timed his drop right. I know I can do that on a fast running Pony/La-7/Typhoon if they fly straight away. Better pilots can do that even when the enemy is maneuvering.
So typically, it is very very probable that this is what happens:
You missed spotting someone, or the enemy purposely stayed away from your point of attack. Then, when you go down low, or start a fight, he'll come by and observe you while you don't notice him. As you time your escape, he is already starting the hunt - you think it's all safe when you extend away from a furball at 400mph, but in truth, the dedicated hunter is dropping down on you at something like 500mph.
You're startled by his presence, immediately break upwards and try to shake him, but the enemy is trained on you, and corners you easily, and shoots you down.
...
There are many, many planes which can easily catch upto the P-51D, given albeit a slight E advantage - La-7s, Typhoons, Bf109G-10s, Fw190D-9s, Ta152H-1(this one, outdives the P-51).. etc etc.
So don't jump down on a furball immediately when you think you have alt advantage. Observe the airspace for a few minutes, and try to calculate the numbers and factors - one especially important factor, is watching closely at the radar bar on adjacent enemy bases - when you're high, the real threat is not any of the ones below you. The real threat is the enemy that is lingering around you in a few minute's distance, or the enemy that has upped from another base, and coming this way.
If you get used to analyzing radar behavior, I'm sure you're rate of success will start going up in notime.
