Touble code po320 please help

I have a 98 durango just went to start it yesterday and it took a while to start it just turned over and turned over then finally started.Went to go down the road it was kinda misssing.Then went to kick in the passing gear and it was like a was brakeing.It would not kick in just sputtering.My check engine light came on so i took it to autozone to have the code read.They pulled po320,Then Multiple cylinder missfires,Cylinder 4 not fireing,and cylinder 8 not fireing.4 codes in all.What should be the problem or what should i change first?I am totally confused on this one.Any info will be greatly appreciated.

what engine do you have and mileage on vehicle and maitenance?? ever change plugs cap rotor wires??

what engine do you have and mileage on vehicle and maitenance?? ever change plugs cap rotor wires??
.....128,546 we have had it for about 3 years plugs and wires about 1 year ago cap and rotor no

motor is 5.2 i think with 128546 miles