97 sunfire, 5 speed, manual; rough idle, hesitation

Hello, I have been having problems with my sunfire, it runs fine when it's cold but when it warms up I start having problems. There will be a rough idle(a thumping sensation in the engine
http://kona.kontera.com/javascript/lib/imgs/grey_loader.gif
(http://www.2carpros.com/forum/here-vp815290.html#)) and it will hesitate upon acceleration. I went to autozone and they ran the diagnostics and it says: Heated Oxygen Sensor (http://www.2carpros.com/forum/here-vp815290.html#), High voltage, and Cylinder misfire in cylinder number 4. I have replaced the fuel pump, fuel filter, distributor cap, temp coolant sensor and BOTH Oxygen sensors. So, I don't know why the oxygen sensors are still showing up on the diagnostics, as I just replaced them. Whats causing this? I don't want to keep putting money into parts that I don't need. Thanks so much for your help!

Whoops, Don't know how that showed up like that.

Let's try that again : Hello, I have been having problems with my sunfire, it runs fine when it's cold but when it warms up I start having problems. There will be a rough idle- it feels like a thumping sensation in the engine

(http://www.2carpros.com/forum/here-vp815290.html#)and it will hesitate upon acceleration. I went to autozone and they ran the diagnostics and it says: Heated oxygen sensor, High voltage, and Cylinder misfire in cylinder number 4. I have replaced the fuel pump, fuel filter, distributor cap, temp coolant sensor and BOTH Oxygen sensors. So, I don't know why the oxygen sensors are still showing up on the diagnostics, as I just replaced them. What do I replace next? What's causing the cylinder to misfire? I don't want to keep putting money into parts that I don't need. Thanks so much for your help!

UPDATE:: I had a friend come look at my car who has worked on cars for 15 years. He had no doubt in his mind that it is the Catalytic converter.

He said that it is in the beginning stages of getting plugged up. I was looking online at Cat cleaner and found Cataclean engine and catalytic converter treatment. Should I try this before I buy a new Cat? The cat is rather expensive and I don't want to buy it if I can avoid it.

I am selling my car within 2-3 weeks and don't want to put more money into it. Should I just buy the Cat cleaner?
Anyone ever used it?

Thanks.