On a truck, "Fx4" indicates that the vehicle is capable of "four-wheel drive." When four-wheel drive is engaged, the power to move the truck is divided equally among all four wheels. Fx4 is typically used in off-road situations, or on snow and ice, rather than under standard driving conditions.
"Fx4" is considered a part-time option because drivers are able to turn it on and off, depending upon the need. This is often done by activating a lever, button or switch inside the cab.
Four-wheel drive trucks are distinguished from all-wheel drive vehicles. In these models, the four wheels are engaged all the time. Though not typically designed for off-road exploring, all-wheel drive vehicles do offer good control under all road conditions.