What does this mean F4WD?

If you have seen "F4WD" then you have most likely seen "S4WD" in the same paragraph. F4WD is an abbreviation for "Full-Time 4 Wheel Drive". S4WD is an abbreviation for"Selectable 4 Wheel Drive." Although both terms refer to different builds of transfer cases. The abbreviations, in my experience, is used in determining what front axle a truck is equipped with.