Difference between 5002 and 5004? I'm no expert, but red in the power reserve sub-dial, brown strap vs. black, different design on the crown, date window (white vs. black). I just like the look of the 5004 more. There used to be issues with the 5004 such as wrong hands and recessed date which has been improved with the versions being sold now. Take a look at SD4K's IWC replica guide at the top of the IWC forum and also do a search for "Best Big Pilot". This is what I did and it gave me enough information to pull the trigger.
You can find trusted sellers under the "Watches, Clothing and Accessories" heading on the home page. I actually just received my watch today and I am extremely happy with the quality. Many say the BP is one of the best out of the box rep values out there. Having tried on the gen version a few times and now having this rep, I would have to agree. I have an Orloff strap on the way (rep strap sucks) and I will take care of the PR issue (indicator not pushed all the way to full power) in the near future to get it as close to gen looking as possible.