Combination of four half-bridge load cells
In order for the bridge to produce an output between S- and S+ the bridge must become "unbalanced" when weight is applied. So if all 4 load cells produce equally increasing or decreasing resistances when weight is applied the bridge will remain balanced and the output will not change. 

If the load cells are arranged such that the left and right halves of the bridge move in opposite directions, then the difference between S- and S+ will increase when weight is applied.