Besides "Welcome" of course, those are some beautiful shots, great detail.
Long, long ago having a Nikon FE (I think that was the model), and an old off road bike, I got interested and would shoot motocross racing where I lived (I didn't race). A lot of it was just kids getting together, some was at a local track, all was great fun. Haven't been to one in ... must be 40 years. I need to see if there are any local ones.
So welcome, thanks for the images, and for a trip back through old memories!
PS. Now get a D800(E) to accompany your D4 and you will be hooked for life.