The best you can do is find the average rate of change over 5 seconds by dividing by 5 seconds: the net change was -4 particles in 5 seconds so the average rate of change is -4/5 particles per second.
If you have reason to think the rate of change is constant, then it is -4/5 particles per second.
(I think abluphoton miswrote and meant "-4 particles/ 5 seconds" which is -4/5 particles per second.)