[greymule]

In most cases, the batter is credited with a home run, even where there's a home run limit and the official ruling becomes, as NCASAump says, a "four-base award."

Now if the fielder was standing a foot in front of the fence with plenty of time and room to make an easy catch, and then the ball went off the glove and over the fence, the scorer might rule a four-base error. Such a ball would of course still not count against the limit.