The Survivors must jump into a sewer hole at a certain point. As an Infected, 
use this to your advantage. While playing as a Smoker or Hunter, hide behind one 
of the large canisters in the room. Once one of the Survivors open the hole and 
are done checking the vicinity, they will jump in. Due to the fact they must 
jump in one at a time or get stuck, make sure you have a clear view of the last 
one in line. Once all three are down and the fourth Survivor is still up, pounce 
or grab him or her. By doing this, he or she will not be able to be saved, 
unless someone throws a Molotov cocktail or pipe bomb.