I had a D200 many years ago, which was my first DSLR (I had used an SLR since the mid '50's). I had a lot of fun with it, but eventually I wanted to do more indoor work and the D200 didn't really cut it. So I got a D3.
It made a huge difference, both in my photography and in my enjoyment thereof. It was a large pain in the wallet, particularly since I went for a new set of lenses at the same time, 14-24-70-200 all f/2.8.
It's not that I take great pictures. The camera doesn't do anything for my color sense or compositional skills or lack thereof. But the pictures were less noisy, the high ISO capacity made the system much more flexible.
The D300 is probably much better than the D200, but the D3 is better than both. And the D3S is better than the D3.
My D3 disappeared a few months back. I used the D200 until the insurance company sent me a settlement. I went out and bought a D4. I did not consider that I had any other choice. And I'm glad I did. Going from a D200 to a D3 was a great experience. Going from a D3 to a D200 was not.
Sorry to impose my story on you, but I'm not a professional. I just like to use the best tools I can afford. And, as expensive as most people consider the D4 + lenses, I think it's worth the investment (it's not a cost). I have a friend that bought a boat. I don't know what it cost him, but it was probably in the $25-50K range. I have another friend who travels to exotic spots to fish and/or hunt. He probably blows $10K on a trip and has only fish stories (and maybe a couple of meals) to show for it. I figure my investment is in the $10K range for the D4, lenses, peripheral equipment, computers, software, backup drives, etc. To me, it's worth it.
The only downside to going for a D4 is that you'll never go back. You'll be using pro gear forever. But then, that's not much of a drawback.
PPS: Now that I think of it, the only other problem with being a D4 owner is that there are very few threads about the D4. 99% of them seem to be about either the D600 or the D800. So you have to do a lot of searching to find something interesting to you on these forums.