o2 sensors or catalytic converter

What is your question?

I have the 4 death lights on. this started when my alternator went out, then I drained the battery getting to auto zone. I replaced both and then I got the 4 death lights and then dealership said I need a new cat for 2007 fj and it's going to cost $3000. another shop says it's the o2 sensors. I don't know who to believe or what to do. thanks