With a negatively sloped output demand curve, this will result in a large reduction in output and hence labor demand. On the other hand, if labor share is large, there is not much room for substituting other factors in its stead when wage rises. Hence the reduction in labor demand will be small. To be sure, the forth law is not stated very precisely anyway. The share of labor in total cost is itself an endogenous variable.
Different producers have different fixed costs.
Changes in labor demand due to birth or death of firms make up a significant part of total job flows.