Yeah, the FI light for just a second is fine- it will stay on if the computer thinks there's a problem.
While you have the spark plug out, maybe try a compression check. If you have a compression tester. Although it doesn't tell you much, it can indicate whether there are leaky valves or worn rings. The 400 has a compression release integrated in the camshaft which limits the compression when it's starting, and that limit can be anywhere between 80 and 120 PSI so the absolute pressure won't tell you anything. The main thing is whether the compression raises after shooting a squirt of oil in the cylinder- if it does, then you're looking at new rings at least.
Anyway, just turning the crank over with a wrench can tell you if the compression is OK- remove the left leg shield (you need to do that anyway to change the spark plug), take off the variator filter cover (2 8mm bolts) and the variator filter (2 screws). Put a wrench on the variator nut (15/16 or 24 mm) and turn it CCW to turn the crank over by hand. It should have a spot every second rotation that gets hard to turn (compressing) then really easy to turn (past TDC and the compressed air in the cylinder pushes the crank back around). It shouldn't hiss while doing so, and it shouldn't be easy to turn through the whole rotation. Again, if squirting some oil in the spark plug hole fixes it, that's a bad thing- new rings. If it hisses and doesn't have much compression and doesn't raise after a squirt of oil, then it's time to look at the valves. It could just need valve adjustment.
If you've determined there's good enough compression, the next thing to look for is some kind of fuel or electrical problem. If the spark plug looks OK, I'd suspect a fuel problem- maybe a badly adjusted throttle position sensor, dirty throttle body, that sort of thing. But since you say it runs OK otherwise, nothing really gross should be wrong with the fuel system.
Good luck and keep us posted! Let's hope for just a dirty air filter or a worn-out plug; when I got my k4 400, my air filter was caked with 1/4" of greasy dust and it had similar symptoms.