So if there are (4) O2 Sensors on a 2000 Ford Expedition 4.6, then how many upstream and how many downstream sensors are there?

There are 4. Do yourself a favor. Replace all of them at the same time. That way you will only have to go to the parts store once and crawl under the vehicle once.

But if you are going to replace only the down stream O2 sensors they are the ones closest to the back of your Expedition.

That was what I was going to do,,,replace both downstream and upstream O2 Sensors. Thanks for the second opinion.

Simple guide... upstream is toward front of car, downstream is toward the rear. He is right, even though the cost goes up, replace all four of them

How many does a 2001 Ford expedition have with a 5.4 engine