If you're fully competent with the D300, you should have little trouble with a D4. The controls are a somewhat different, but that's mostly placement and/or settings that can be changed. Photography doesn't change - this isn't a radical thing. If you were a beginner with a D40, you might find the change quite disruptive, but even there it's a matter of learning the (many) controls.
From a D300, you should find that the AF is slightly improved, the high ISO performance is noticeably better, and the whole system is more responsive. These are hardly things that should overwhelm someone that's already doing bike racing and air shows.
What's (an) airsoft?
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!