As always, there are arguments for both sides.
The main reason people prefer a 4-prong setting is the fact that there is less prongs/metal surrounding the diamond, making it stand out. A 4-prong setting shows off a little more of the diamond and really can pull off a nice look. However, this is the only real advantage to a 4-prong setting.
Another argument commonly brought up is that with less metal surrounding the stone, more light is able to enter into the diamond thus making it brighter and causing the diamond to sparkle more. This is only partially true. Obviously more light will get through to the bottom and sides of your stone, but this will not have much of an effect on the diamond’s brilliance. Only light which travels in through the top of your stone is reflected back out through the top of your stone. The light that reaches the bottom or sides of your diamond may brighten the stone slightly, but not significantly.
The major disadvantage to a 4-prong setting is the risk of losing your diamond. If you happen to bend just ONE prong back your diamond will likely fall out. With the 4-prong comes great risk, so I would advise anyone who values security over a slight change in appearance to go with a 6-prong.
As I just said, the big advantage to a 6-prong setting is the safety factor. You will almost never lose a diamond set in a 6-prong head because even if TWO of the prongs break completely off (which rarely ever happens), you still have four left to hold the stone in.
You may have a little more metal around your beautiful diamond, but it can be a lot more comforting to see that than to look down and see your $5,000 diamond is missing.
Regardless, you should always have your diamond ring insured. However, do not insure it at the jewelry store where you bought it. Go to your insurance company, and this way you’ll be covered for anything that happens to your ring. Read more about In-Store Warranties and Insurance Policies.