1994 Isuzu Trooper O2 Sensor

I had the O2 sensor replaced on the above car. However the "Check Engine" light stayed on. I was told that there are 4 O2 sensors or the car and that another one must also be bad. I took the car back to have the problem fixed. The "Check Engine" light came back on after only a few minutes of driving. Are there 4 O2 sensors? If so would they all go bad at the same time? How much should it cost to have one replaced?