I was under my truck this weekend and verified that there are 4 O2 sensors. With the PaceSetter Long Tubes, do they have 2 or 4 bungs? From trhe picture it likes only 2.
The two O2 sensors closest to the engine, they are called the upstream sensors. They are the important ones that you must retain. They give the ecm input for a/f ratios.
The two O2 sensors after the catalytic converters are called the downstream sensors. Their primary purpose is to monitor the efficiency of the catalytic converters. On most vehicles that is their only purpose, but I've read that on chrysler's they have a minor impact on a/f ratio input as well. Keeping these two sensors is sort of optional.