GMC 2005 Sierra 1500 Pickup, Owner'S Manual, Page: 309 / 574

4: In this mode, all four wheels will help steer the
vehicle. If you want to use 4, and your vehicle is not in
this mode, press the button, until the 4 indicator,
located to the right of the 4-Wheel Steer button, comes
on and stays on. If the 4 indicator is flashing, you
will have to center the steering wheel by turning it to the
left or right.
At slower speeds the front and rear wheels will turn in
opposite directions. This helps the vehicle make
tighter turns, such as during parking, cornering and
turning into tight spaces.
At higher speeds the front and rear wheels will turn in
the same direction. This improves stability of the vehicle
during lane changes and sweeping turns.
Slower Speeds (below 40 mph (64 km/h))
Higher Speeds (40 mph (64 km/h), and above)
4-15