The U.S. National Hurricane Center says the storm is now packing sustained winds of 160 mph (260 kph) and it could threaten the Baja California peninsula next week.
Authorities in the resort of Acapulco closed the port to small craft after Rick kicked up heavy waves and gusts of wind.
Rick was centered about 290 miles (470 kilometers) south of the port of Manzanillo, west of Acapulco, on Saturday afternoon and was moving northwest near 15 mph (24 kph).
Forecasters expect it to stay well off the coast for several days before bending east over cooler waters and hitting the peninsula as a Category 2 hurricane sometime Wednesday.
MIAMI (Oct. 17) - Hurricane Rick quickly strengthened into an "extremely dangerous" Category 4 storm off Mexico's Pacific coast on Saturday and forecasters said it could strike the Baja California Peninsula in about five days.
The storm had sustained winds near 145 mph and it was expected to grow into a monster Category 5 storm with winds surpassing 155 mph, the U.S. National Hurricane Center in Miami reported, though it said the storm was likely to lose much of that punch before hitting land.
Rick was centered about 280 miles southwest of Acapulco Saturday morning and it was moving west-northwest near 12 mph, the center said.
Forecasters said it was projected to stay well off the coast for several days before bending east over cooler waters and hitting the Baja California Peninsula by early Thursday as a weakened Category 1 hurricane.
Meteorologist Jessica Schauer told The Associated Press that warm waters fueled Rick's rapid jump from Category 1 to Category 4 in only about 36 hours.
"Right now it's over very warm water and the current forecast track keeps it over warm water for quite a while," she said.
Rick was forecast to pass near Socorro Island, about 300 miles south-southwest of Cabo San Lucas, on Tuesday. The island is a nature reserve with a small Mexican Navy post and it hosts scuba diving expeditions in winter months.
Acapulco's Civil Protection Department had earlier issued a warning that rains from outer bands of the storm could trigger landslides and flooding in the resort city.