ok, the car is often hard to start when it has been sitting for 6+
hours. It stumbles and will sometimes even stall when placed into drive
or reverse at first. Even with the car warm, it sometimes will stumble
when I accelerate. I have changed spark plugs, oil, oxygen sensor, and
fuel filter...also my air filter looks fine. My service engine light
usually comes on when im driving between 80-100km/h and sometimes if i
keep the speed steady at around 50km/h. Can someone plz point me in the
problem...thank you for your help.

Check your EGR valve, very often this kind of probleme came from this.

Search on this forum, many post talk about egr valve

I have checked the EGR valve, Coolant Temp sensor, and Air mass. I have
changed the O2 sensor, but the problem has still not gone away. It only
occurs when the engine is hot, and its as though the engine is
being flooded. Diagnostics says "4-4" or "Oxygen Sensor or circuit -
lean exhaust detected". The O2 sensor is brand new, and I have checked
the connections, so im back at square one. Any help would be greatly
appreciated, thank you

hmm..how about a bad cat?

Have you replaced any or all of your vaccum lines? Usually a "stalling" of this nature can be resolved this way.