If it's a misfire first thing you should do a compression leak down test. I am just gonna guess on this one but seems like the good ole #4 has a problem. I have heard about a problem a couple times already where during compression testing bubbles will be seen in the coolant reservoir. The bad news is that either the head gasket is leaking internally or the head is cracked. Sometimes this is visible by inspecting the spark plugs and seeing which one has been getting coolant on it.
If it's just hesitation check the operation of the throttle position sensor. Good luck