A "kinematic" connection to ground is one which provides no additional stiffness to the structure. That works perfectly when there are 3 points touching the ground. A body has exactly 6 degrees of freedom in space, so you need to create exactly 6 constraints. With three points, you can do that by fixing one point (A) in XYZ a second point (D) in XY and a third point (C) in Y leaving all rotations free. That is exactly 6 constraints. If you have 4 wheels in contact with the ground, then the ground is effectively contributing to the stiffness of the frame. If you support a fourth point ( in Y, that is one extra constraint for a total of 7.
When you applied Fixed Support to four points, that controls both translation (XYZ) and rotation (RxRyRz) and is 24 constraints, which is way too many.
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