The comparison that a baseball fan could make is that Hamilton did the hitters equivalent to a pitcher throwing a perfect game. Both feats are incredibly rare, as there are just 21 perfect games in MLB history.
That brings up a different question. Which feat is more impressive? A perfect game or hitting four home runs in a single game?
I tend to lean toward the hitter in this argument, and here's why. In baseball, the pitcher dictates everything before the climax of a sequence. To this, the hitter has no idea what's coming about 90 percent of the time. That would lead you to believe that the act of hitting one home run is difficult.