The d4 is a wonderful camera for low light, fast frame count and lesser post processing than the d800. The d4 is not too much camera for you if you have been using to the d300.
One thing to consider if you need reach, the d4 will not have as much as the d300 with its 1.5 factor. It always seems to me that I never have enough focal length of shooting birds, but the low noise at high iso with my d4 makes it a natural during the long cloudy and rainy weather we get here in Western Oregon.
Once you get the d4 set up the way you like and with any camera there is a short learning curve, the camera will just get out of your way and you can concentrate of the subject.
I do think you would be surprised after shooting with the d300 the exceptional low light quality of the images. One beautiful camera.