Guys, I think that the 4 O2 sensors are for the California market, the 2 O2 sensors are for the rest of the US. I just bought a 03 4.7l extended cab 4X4 and noticed that I had empty sockets in the fuse box for O2 sensor circuits. I found out that the 4 O2 sensors are a specific emission package, I assume for the CA market.

 

^^^ ahhh thank you for the info! good point