During my presentations to golfers, especially new golfers, I emphasize the importance of yelling, "FORE!!!" if an errant shot may hit someone.
A 69-year old man in Southern California was hit on the head with a golf ball, and nine days later he died of the injury. It's a serious matter to yell fore if someone may possibly get hit, and it's important to protect your head if others are yelling fore near you.
If you're part of a foursome and the player doesn't yell fore when he or she should, then you should shout it if you think someone is in danger. And, remember, to really shout. It does the folks in danger no good, if only those on the tee can hear you.
Be careful and play smart!