To jump 3 to 4 times higher than normal you'll need to find a vehicle which has a front bumper that you can stand on. Jump on the bumper, sprint toward the driver's seat and jump. If done right you can get onto high up buildings that you can't normally get onto (no skyscrapers or anything that high). The average height for me is about 1 - 1 1/2 stories high. The car that I usually use is a Nordberg (a jeep-like SUV).