I got 2 codes this weekend, I was wondering if anyone knows what this means I got them cleared, but I was wondering what the second is?

PO421-Cylinder 4 misfire--taken care of new plugs/wires
POXXX-Catalyst value returned low--bank 1 of 2????

Do you think its my cat?--the car has a lot of miles on it (highway)
I have a '00 4 cylinder fed spec--