When weeding on all fours, the goal is keep your whole spine long. Because you need to shift around while you work, keeping a long spine requires watching and feeling how your shoulders are bearing responsibility for reaching, grasping and supporting your body weight. Also, be aware of any pockets of muscle tension along your trunk. Once you become attentive to unnecessary tension, you can relax out of it.
A long spine goes from the top of your head to the bottom of your tailbone, and includes the pelvis. Try imagining that your spine is a garden hose -- long and flexible.