White iphone Home button defect?
I can feel the edges of the button on both my black iPhone 4 even when the button isn't depressed - so your description alone doesn't necessarily mean there is a defect (but you should be very happy with your new purchase and it could very well be a defect). 

If you can go to any retail (cell carrier or Apple) store and compare your device to others (or have friends with any iPhone 4 to compare) it will be very easy for anyone to agree if your phone's button is different than the rest. No matter where you bought the phone - Apple will exchange it if the hardware is defective at no cost.

You also can return a new phone to the seller within 30 days - but getting it in the hands of an Apple tech will be the fastest way to know it's covered or a defect - or if it's just a normal variation of the manufacturing process.