What was your aim with the flash? To brighten up and stop action a bit better (it looks pretty dark in there otherwise) or just to give a distinctive look? If is just to help illumination a little a powerful speedlight on camera or a couple off camera at 1/8 – 1/16 power and a little negative exposure comp dialled in can give good results and will allow for fast frames per second. It may be difficult with such a fast sport but for slower things it can work pretty good actually. I don’t know what the D4 manages as a fastest shutter speed with flash but on my 1D I’ve used it a couple of times to decent effect but then that manages 1/400 as highest shutterspeed with flash. It doesn’t give enough light to light the scene but it will bring out whatever you are pointing the camera at out from the background and it’ll make the background seem darker. Obviously usual caveats of direct flash apply but small amounts of fill can look good.