That Briggs 4.5hp motor should take about 20oz. of oil. There is likely an oil drain plug on the bottom side of the motor toward the back of the pressure washer. Lean the washer forward and look at the bottom side. You can remove it by using a 3/8" socket extension without a socket. The plug has a 3/8" square hole in it.
EDIT...
It appears that the engine does NOT have a drain plug. The manual says to tip it over and pour it out of the filler tube. What a joke!
It does take about 20 oz. of oil though. I'd say you don't have enough oil.
Pressure Washer Manual