We alo take MotoCross, Running and other Event Photos. The reason I set my cameras for only 4fps is that I hate taking double shots when I don't want to.
I use AF-C use the AF-ON button or program my AE-L/AF-L to AF-ON instead of pressing the shutter half down. This keeps the Nikon in constant focus. I am therefore ready for a second and/or third shot if needed.
I also keep my eye on the ViewFinder for much of an 8-10 hour day. Very tiring but I miss few shots, and like I keep telling everyone, you get what you see.