Wildlife photography

 i would take the d4 since its a powerful masjien that delivers 10-11fps where the d3x is around 5 fps, the low light capebalety on the d4 is way better than the d3x. the d3x revolution is higher than the d4 so if you want higher revolution take the d3x but remember that when shooting at high speeds in low light conditions will not be posible with the d3x since its iso rance is 100-1600 where the d4 offers you 100-12800 iso with useable pictures up to about 6400 iso.