Set the drag on your reel at 1/3 the breaking strength of the line and it's quite easy to land a fish that's only as big as the line strength.
4 pound line records, as of 2009:
Greater Amberjack - 26 lb, 8 oz.
Dolphin (mahi mahi) - 52 lb, 14 oz.
Black Marlin, 152 lb, 1 oz.
Striped Marlin, 218 lb, 4 oz.
White Marlin, 97 lb, 4 oz.
Tiger Shark, 159 lb, 0 oz.
Yellowfin Tuna, 55 lb, 1 oz.