2010 Nissan Altima code P0420

Was driving my car home from work last week and
it popped the Service Engine light on with code
P0420, referencing the catalytic converter. Here is
full description: The Catalytic Converter takes the
exhaust gases left over by the combustion process
and converts those gases to harmless emissions.
The engine control module (PCM) compares the
front and rear oxygen sensors to each other to
determine catalytic converter efficiency. If the input
from the rear oxygen sensor starts to match the
front oxygen sensor, the PCM will set code
P0420.Catalytic converters are covered under
federally mandated 8yr/80,000 mile warranty by
the vehicle's manufacturer. Catalytic converters
usually fail as a result of something else (engine
misfire, internal oil leak, or internal coolant leak)
not being repaired. The original cause of the failure
is usually not covered under warranty.
Wondering what might be a good idea to fix the
issue. Auto dealerships wants $4k to fix it and
come back with a list of other things supposedly
wrong.
The car has less than 100k miles, so not really
looking to replace it quite yet