You sell yourself short. If you can handle a D300, you can handle a D4 with only a minor learning curve.
Tired of waiting for a D400 that may never come, I purchased a D4 three weeks ago. Having used a D300 for 5 years, I can tell you without reservation that the transition to a D4 is not a problem. As was mentioned by others, the D300 menus are almost identical to that of the D4. The few differences that do exist with the menus or buttons are easily mastered.
I love my D300 but the issue that continually frustrated me was it’s relatively poor ISO capability. As a wildlife photographer I was frequently forced to sacrifice image quality to capture a subject in low light, particularly when I wanted a high frame rate for subjects in motion. On the other hand, as a landscape photographer, It’s hard to overlook the 36MP D800.
As a wildlife AND landscape photographer, I wrestled for months regarding whether I should get the D4 or D800. Both are excellent cameras, each with some unique strengths. I made my decision based primarily on five personal requirements.
1. High frame rate 2. Fast, accurate focus 3. High ISO (low light) capability 4. High dynamic range 5. Bullet-proof construction
The D4 met all of these requirements with flying colors. While the D800’s 36MP is a very compelling feature for landscape photography, I feel the D4’s “measly” 16MP is plenty for my needs given that I’ve yet to have a problem generating interest for the A2-A4 size landscape photos taken with my 12MP D300.
In the 3 weeks I’ve had the D4, the camera has simply amazed me. While costly, I have not regretted its purchase for a minute.
Given your interest in photographing motorcycle races, airplanes, your needs for a higher frame rate are even greater than I require for photographing birds in flight. If your requirements are similar to mine and you can afford it, you will find no better camera than the D4 to capture motorcycles, airplanes and wildlife at high speed in all sorts of lighting and weather conditions.