Ditto to all the above. I moved from a D300s to a D4 and had no issues making the jump. I'm an amateur and my sole justification was 2 kids in sports (basketball & swimming). Needing (wanting ) the speed and low light capability in poorly lighted venues, the D4's been great.
No mistake - the D4 has alot of capabilities that I haven't used yet. But that'll give me something to work on - the kids'll graduate sooner or later!!