What's the difference between 3.8", 4.0", 4.6", and 5.0" fat tires?

Tire width directly affects flotation and rolling resistance. 3.8" is the minimum for fat bike classification—lighter and faster but less flotation, often accepted on groomed trails with width limits. 4.0" is the most popular all-around size, balancing flotation and rolling resistance. 4.6" provides noticeably more flotation for deeper snow and softer sand but is heavier and slower. 5.0" is for maximum flotation in the deepest conditions but is heavy, expensive, and may exceed trail width limits. Most riders are best served by 4.0".