I wouldn't just throw parts or suggest quick fixes to get rid of the code. The P0420 code is merely a symptom that truck is experiencing, and isn't telling you the root cause. I'm no mechanic, but know many people that are, and their advice regarding this code has always been to diagnose from the top down. I also want to advise you that if you do any work to your car, it is at your own risk. If you don't feel confident enough, take your truck to a good mechanic.
There are a lot of things that can lead to the code, but may not necessarily trigger it. When I decided to tackle this problem, I started from the simple stuff first. Check the battery connection and ensure the that MAF and TB is clean. Check the spark plugs/ignition coils and wires to ensure your truck isn't misfiring. Is the truck burning oil? Combusted oil is a common culprit for a damaged cat, and over time, the a P0420 will be triggered.
Running too rich or lean can wreak havoc on the catalyst system. Check for any vacuum leaks, and ensure that things such as the PCV valve is in good working order. After all that, check for any exhaust leaks. If you encounter and fix these issues you may even find that your 4runner is idling and accelerating better, and also not throwing off any air fuel ratios that may be confusing your O2 sensors. A fuel system check can also determine if your truck is running too rich or lean, so you'll want to ensure that your pump, fuel filter, injectors, fpr are in good working order. After all this, I would then move onto checking the catalytic converter and O2 sensors, which I won't get into too much detail.
I can't remember off the top of my head what the proper temps for the cats should be, but you can have someone rev the truck to 2500 RPM and monitor the temperature
of the cats. The temperatures should fall within a certain range. This can be done with an IR thermometer and the help of a friend. It's also a good idea to check if your exhaust smells like rotten eggs or if your cats rattle. Once you narrow it down to your cats or O2 sensors, then that's when I'd change them.
I know it seems like a lot to diagnose one simple code, but I come from the camp of being thorough, and not just throwing parts at the problem (you may be throwing in the wrong parts and wasting money). A proper diagnosis may take longer, but taking the steps to solve the problem and everything leading to it is worth it to me. Quick repairs just to get rid of the CEL doesn't solve anything, and I hate knowing that my vehicle could be running better from doing a proper repair.
