Per Ford:
4A (4X4 AUTO)
Provides electronic controlled four-wheel
drive with power delivered to the front and
rear wheels, as required, for increased
traction. The system continuously monitors
road conditions, driver's input as well as
other vehicle sensors and optimizes the
system's behavior based on terrain mode
selection.
Weather Mode - For less than
ideal road conditions, such as
snow or ice covered roads.
Weather mode inspires confidence without
taking away from driving pleasure. Weather
mode automatically engages 4x4 Auto,
lowers throttle response and optimizes
shifting for slippery surfaces.
Link to above:
http://www.fordservicecontent.com/Ford_Content/Catalog/owner_information/2017-Ford-F-150-Raptor-Supplement-version-1_su_EN-US-EN-US_10_2016.pdf
And here's a description of 4A / Weather Mode
The throttle is muted, meaning the truck won’t accelerate as hard when you put your foot down. The idea is that this will help you avoid spinning your wheels into oblivion while you’re stuck in snow. The transmission shifts short to keep engine speed as low as possible while keeping you moving forward. Traction control operates on a heightened aggressiveness. Basically the system is quicker to cut power if it detects slight slips or yaw.
The 4WD system is set to “Auto” here which allows the truck to behave like some AWD vehicles, sneakily sending power to front wheels when sensors decide that can help the traction situation.
This is the only driving mode where auto start/stop is active other than “Normal.”
