Should I replace all 4 O2-sensors?

Or just replace those in front of the cats? In this case can I put those that are now in front to the rear of the cats? They are "only" 40'000 miles old. If they are slow they are bad for fuel metering but maybe not to check the cataysts function (I hope so).

I researched about ethanol in fuel in Europe but I haven't find any information. Maybe they do it...
Do you see a difference in fuel trims when the fuel contains ehtanol?
In the USA they put ethanol in every kind of fuel? Every grade? Or just in the grades with a low octane rating?