Both mine and my wife's iP4s have a little rattle in them. Hers more with the volume buttons but both phones rattle a tiny bit with the sleep/wake button on the top. My guess is that is the button that gets used far more often than the volume buttons so it gets the most wear. I don't think it's a design issue because if the phone was made of plastic you would't hear it. This phone is made with glass and stainless steel so the tolerances are going to be different.
I think this is more OCD than anything else but just to be sure, I held down the sleep/wake button and gave my phone a small jiggle and the rattle stopped. I only noticed it when I was using the Bing app of all things trying to get the voice search function to work.
The only issue I've had with my iP4 was a bad pixel in the camera itself--basically, all of my videos had a stuck blue pixel in them. Grrr.... On the plus side it took less than 10min with an appointment at the Apple Store to get it swapped, the part that took the longest was they wanted to detail exactly where the pixel was and everything, my guess is to keep track of defects. I don't recall this when I had my iPhone 3G and had it swapped for frying my SIM card.
(Incidentally, I did discover that if you are having trouble getting the voice search function in the Bing app to work, give the phone a tiny jostle and it will work--I presume it is because it doesn't want to turn on the proximity sensor until it notices you motion the phone up to your ear.... cool yet unnecessary design."