which code should should i work on first? po496 po442 po420 po455

I saw your other question about the gas fumes smell in your cab, and three of these codes are related to the EVAP system. That system is designed to recover fuel fumes from the gas tank and return it to the tank, either you have a cracked housing in the EVAP canister or some other defect, may even be the gas cap. The filter material is supposed to last the life of the car, so that isn't likely to need changing, but the housings do fail. The other code is related to the catalytic converter, and O2 sensors, listed among the causes for this code are leaky injectors, which was my suggestion in your other question posted earlier. It can also be set by an intake or exhaust leak, but I still tend toward injector leak (or leaks). Either way, your car is in need of servicing pronto.