The first thing is, I would recommend against a full-flaps flat-turning contest. Yes, the Corsairs all have a tighter turn radius at full flaps, however your turn rate is sufficiently lower than the Spit's that unless you can get the shot within the first circle or two, he's going to get around on you eventually.
Use high and low yo-yos instead, especially the low-yo, which you can use to build your turn rate while still cutting inside his corner if he tries to flat-turn against you. Use a fairly high-aspect yo. In the case of the high-yo if you're positioned properly you can just roll over the top of him, especially if you kick hard on the inside rudder (IMO good rudder management is even MORE important to the Corsair than the flaps). And if it leads to the rolling scissors, the Corsair can use that to great effect.
Otherwise, remember your main advantages in the -4 are raw power and speed. Even if the Spixteen has a starting altitude advantage, the -4 is fast enough and accelerates well enough you should be able to extend and reset the fight with only a shallow nose-low attitude. You can also use the standard energy saving/building tactics you would use against any higher opponent (my favorite is a shallow nose-low turn to build E and bait him into an attack, followed by a sharper break under him as he closes for a shot and then a quick zoom climb back up to grab a bit of altitude. Lather, rinse repeat until either you've neutralized E states, or you've gained position to roll over the top of him).
Oh, and don't worry about getting saddled up. Take whatever shot he presents, even if it's a quick crossing shot. Spits don't take damage particularly well, and even a brief snapshot can pop his wing off.
