2000 Hyundai Elantra with 129000

I have a 2000 hyundai elantra with 129000 miles on it. The check engine light has come on with the codes PO300, PO301, PO302, and PO304, what exactly does piston misfire mean, and the dealer was suppose to change the crank shaft sensor today, does that have anything to do with the misfire, also they reset the timing belt last week, without changing the sensor, does that cause the code to come up, I have spent almost $2000.00 up to this point, what else do I need to do, and how much more is this going to cost, or do I need to take it back to the dealer and have them fix it and cause another stink? I am at my wits end someone help me!