Carefully apply upward pressure on one of the motor arms while bending the body downwards to snap the lower body armpice out of place.

Repeat this step for all four arms. The lower body will separate from the main frame after snapping the fourth arm out of place.

You may need to bend the arm quite a bit to snap it out. This is a durability feature that allows the X4 to "crumple" in a crash without breaking.