I've never shot such a MB race, only road races, but from that I know that "head-on" is the most likely type of shot to be disappointing because (take your pick);
they may often lack obvious 'action'; they may lack motion blur of legs, wheels or background
the AF, or your timing of shutter release at manually pre-focussed distance, may go wrong
the subject merges into a sharp background
That said, due to the angles of the riders, we do get a sense of action with your shots - #2 is my pick of the bunch.
I don't disagree with the comments and suggestions from the others above, but I wonder if there was any possibility of getting side shots? Perhaps "forestry" is just too dense to contemplate successful focus.
I can see the reasons for the differing WB, the sun was shining in #2 and #3, where as I suspect #1 and #4 were shots while it was hiding behind a cloud and this does affect the WB, you could try nudging it (falsely) towards something warmer and see what it looks like.
Personally, I would have used higher iso than 160 tops, to get a faster shutter speed (when necessary, but be careful of freezing too much, especially when head-on), perhaps you just didn't need to.
Hope that's helpful,