Which O2 sensors?

I have a 2000 XJ with about 96K miles. I would like to replace the O2 sensor, but just found out there are 4 of the dumb things. Is one of them more important to mixture than the others, or do I need to replace all 4 to get maximum benefit. Also, do I need to go with OEM or would Bosch work just as well.

Thanks,

Eric

Replace them all. Bosch is fine.