Correlating Ki values of pesticides to bacterial growth
Ki is the equilibration dissociation constant. The smaller this constant is, the stronger (more specific) is the binding between the inhibitor and the enzyme. 

If the residual concentration is now higher than the Ki, for each molecule from the inhibitor which dissociates from the enzyme a new will bind (simply because of the concentration). If the concentration is lower (or much lower), this will not be sufficient to inhibit the enzyme. This assumes that the relation between enzyme, inhibitor and substrate are linear (and we are not getting some kind of saturation here).

Under the conditions mentioned above, P1 and P4 will inhibit the enzyme. For P2 and P3 the concentration is to low.