One thing that I did NOT see in reference to your question was the issue of LENSES. Depending on which lenses you already HAVE with your D300 (DX, FX, "film-era"...), you may be in for additional expenses with a D4. My first "quality" DSLR was a D200, and I purchased a few DX lenses. Of course, I still had a number of "legacy" Nikkor FILM era lenses. When I purchased a D3 (and just upgraded to a D4), I found that while you can "use" the DX lenses on a D3 or D4, it comes at a SIGNIFICANT reduction in resolution (MP drop from 12-->~5). So, I eventually upgraded to primarily FX lenses. Of course, you can do this "incrementally"; but you need to realize that (unless you have a lot of 35mm AF lenses) using your current DX lenses will not be a satisfactory long-term solution.
Just wanted to make sure that you understood the LENS issues with going from DX format (your D300) to FX format (D4).