I’m not 100% up to speed with Nikon’s camera range, it seems like you have to go pretty high up to get decent 4K for more than a few minutes in any decent quality. If you want to shell out for that you would be looking for something like a D850. Alternatively you can look at getting the Sony A7 or A9 mirrorless camera to use your Nikon lenses with but the results will be mixed depending on what type of Nikon lenses you have.
