How many oxygen sensors does a 99 Mazda millenia have and where are they located?

I have a 99 millenia myself and it has 4 (four) 02 sensors, and each go for around 200-450 in price. I don't really know where they are located though, but i do know they can cause your check engine light to come on and flash from time to time, as well as the car hesitating and feeling like it is going to stall.