That's a bit unfair since a duel doesn't really reflect the MA environment, and the Spit V is a beast of a dueling plane. Under very restricted dueling rules, the Spit V holds serious advantages against K4s, La7s, F4Us, etc. This is particularly true if you know that the other guy in the F4U is very likely to be dumping speed/dropping flaps/using air brakes to gain quick angles after the merge. In the MA environment, most of the time you don't know if that guy's gonna hit the brakes or keep on going.
Spits aren't "fairly easy" to fight, but many planes are highly survivable against them. You might not get a kill against a well-flown Spit in a 109K4 in the MA, but the odds are very unlikely that a co-E or lower E Spit will turn the tables. The K4 should be able to dictate the fight, put the Spit on the defensive, and egress if necessary.
I disagree with all the evaluations thus far...
* The single most important factor is the initial e states of the two engaging planes, against anyone worth their salt, the initial situation and 'e' states will make all the difference.
* In regards to the 'k4' vs the spit 16/8/9, if they enter at the same 'e' state, given equal sticks, the spit16 will win most of the time. Why? Guns with better trajectory, better turn, very similar climb rate. Furthermore, spit 16 accelerates faster in a dive, and if at merge they are co-e, well then the spit can get the shot off long before the k4 is at a higher e state if the k4 chooses to climb away to bnz.
* The analysis of the spit vs f4u is mostly true, except I would argue a well flown f4u doesn't need to be at the same 'e' state, again because of the guns. I have seen many 'expert' toon pilots in f4us make shots off from 600+ out with 50 cals, so as long as they can get their nose in a position to take a shot they can get you. Also, I would argue that the f4u-4 is the best prop plane in the game and will be able to maintain 'e' with the spits.