I went from a D300 to a D4 (and D800e). For still images the changes are minor. You have more options for the programmable buttons, many of the menu choices have expanded, there are a few more dedicated buttons, and some of the settings have moved. I'd consider the changes evolutionary and easy to understand. With my D300 settings spreadsheets in front of me I easily stepped through the D4 configuration menus.
Of course the D4 adds video. I have not used video, but obviously there is a learning curve there.
Compared to the D300 the D4 has faster frame rates, much better high ISO performance, greater dynamic range, and improved AF. I'm an amateur who, like you, was in the position to buy a D4 and did it. I had no economic justification, but I never have regretted the decision.
Gary in SE Michigan, USA. Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery