Code P0420, engine warning 4LO blinking and Slip Warning

Hello,

Since you have changed the sensor with no change to the outcome, it is a possibility that the catalytic converter might not be performing efficiently.
As the Prado is not sold in the US I don't have the exact diagnostics, however it should be similar to the Landcruiser that sold in the US.

Here are the probable cause for that code:

DTC
P0420
Probable Causes
Air Fuel (A/F) Ratio Sensor Bank 1 Sensor 1
Front No. 2 Exhaust Pipe - Three Way Catalytic Converter (TWC) Front Catalyst and Rear Catalyst
Gas Leak From Exhaust System
Heated Oxygen (HO2S) Sensor For Bank 1 Sensor 2

I have include a diagnostics procedure for catalytic converter for your reference:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter
https://www.2carpros.com/articles/bad-catalytic-converter-symptoms
https://www.2carpros.com/articles/catalytic-converter-replacement

Please let me know if you have any questions.

Thank you.