To use four-wheel drive in most trucks, simply shift the small four-wheel drive lever into the appropriate gear. Four-wheel drive should be used when road conditions require more traction than normal, such as during snowstorms or in heavy mud.
Some four-wheel drive vehicles, especially older ones, have manual-locking hubcaps. These vehicles require you to stop the truck and get out to turn the locking mechanism by hand. Most newer vehicles have automatic-locking hubcaps so this step is unnecessary, but they may need you to put them in reverse when you shift out of four-wheel drive in order to unlock the hubcaps. Always shift back to 2H after using four-wheel drive because using it in normal conditions can cause damage to the drivetrain and reduce gas mileage.
Most four-wheel drive vehicles have three options: 2H, 4H and 4L. 2H is for normal road conditions. 4H is for faster driving, such as driving on an icy road. Many manufacturers recommend staying at or below 55 miles per hour when in 4H. 4L is usually used only for off-road driving. It gives the most traction but is designed to only be used at very slow speeds, usually less than 40 miles per hour. Use it if your truck is stuck in mud or snow, or you are driving in steep and slick off-road terrain.