I had the 300 f/4 AF-S for about 15 years...a VERY fine lens...autofocus is good, but not stellar; read the Thom Hogan review for a complete rundown on it! I had the 300/2.8 AF-S Mark II (the lightweight magnesium barrled model) concurrently with the f/4 lens for a decade or so; it focuses MUCH faster and MUCH more-emphatically than the 300 f/4 does. Still--the 300/4 is good, plenty sharp, and was the lens I used much more-frequently because it CARRIES so easily in a nylon sling pouch.
Works decently with the TC14e or TC14e-II for a 420mm f/5.6, but the focus DOES GET "nervous" when the 1.4x converter is added. Still--even with the 1.4x added, this is sharper than many long zooms.
This lens is a good focuser, but not 99.5% reliable on fast action, like the 300/2.8. Bur for most uses, the 300/4 is pretty good. Sharp, easy to carry, easy to stow, light in weight. It's hard to explain the focusing of this lens: jittery is one word some people might use...it's not bad, but it's perhaps something like 95% focus-reliable on fast action; it CAN hunt a bit at times, or drift a bit, which is different from the 300/2.8 or 200/2 models which are more like 99.5% focus-reliable on fast action.
What I mean is: take the 300mm f/4 AF-S to a track meet and shoot 800 frames with it: there will be 5% of those where the focus might not be spot-on...or 40 frames that could be at issue...something along those lines...