Just Japan and USA got 4 o2 sensors. Everyone else made do with 2 I think.

I think (think) the engine is OBD compliant enough to show what the o2 sensors are doing with a bog standard code reader. With the engine running they should show the voltage swinging up and down. If they're not doing that you know there's a problem with the sensors somewhere.... or something is causing the engine to constantly run rich.

I know where you can get some cheap CATs if it is them.....