speaking from no personal experience as I shoot Canon: Two great sites I got some of the info from.
http://nikonites.com/d800/4766-d800-vs-d4.html
http://snapsort.com/compare/Nikon-D4-vs-Nikon-D800
The big thing is what are you going to be shooting? Are you just wanting a nice SLR to get family and pet pics or do you plan on using it for sports, wildlife, etc with the hopes of selling or going pro?
Important points:
D4 can shoot 10-11 frames per second. D800 can shoot 4. So basically if you hold the shutter button down for 1 second you will end up with a burst of 10 pics from the d4 vs 4 from the d800.
D4 does not have a built in flash d800 does. The flashes tend to be crap though and most will never use it without a diffuser.
d4 does have a longer battery life, but with the d800 you can solve that fairly easy with battery grips.
d4 is larger and weighs more than the d800.
They are both full frame sensor, can HD shoot video, have live view mode, and have hd output.
Personally: I'd say unless you already an established sports photographer, Id go d800. Its going to cost you less and be a little more beginner friendly but still do more than enough for you to have fun with it.
Picture quality is more about the glass than the camera!
Invest in glass. It will hold its value vs a camera and then when or if you do upgrade the camera you will still have nice glass.
Again lenses you need will greatly depend on what you are shooting. I have 3 lenses in my bag right now, about to get a 4th. I have the kit lens that came with the camera, a 75-300 zoom that I use for all of my autocross shots, and a nifty fifty 50mm 1.8.
The 50mm 1.8 is a great portrait lens and is a great starter lens because it is under $100. You can spend more and get the 1.4 but that is up to you.
18-55mm f/3.5-5.6 DX is another great basic lens that will cover most day to day shooting.
A great option is to get on cameralensrentals.com and rent the lenses and camera bodies you are thinking about for a few days. Its a great way to test it out and see in person which you prefer. You may find you need more zoon lenses instead of prime lenses or maybe the other way around!
However, dont think that you need to drop a huge amount on a camera and lens combo! I shoot on a Canon Rebel XT that is at least 5 years old. It is my baby, and has served me well. I am currently saving up for 2 things though, a MarkII camera (canon's pro series) and a 100-400L lens (super telephoto, great for track events. but stupid expensive.)
Example pics: I do all sorts of autocross and car pics. My business name is Pointer Cone Photography and you can check out my smugmug at jordidunn.smugmug.com
Taken with the XT with my old 75-300 the before it died (this was after it started giving me trouble):

Porsche mod car

Coned...

The cops came out to play with us at a charity event.
So not bad for a cheap lens and an "out of date" camera. But here is what happens when you put pro-glass on the end of it. These were taken with a 100-400mm L lens It was great! I love that lens but the cost is an issue.

Very crappy rainy day.
