My 4-runner has the low tire pressure light on.

put it at the correct pressure as defined on the door jamb and press the "tire reset" button...so the next time you're about to have a BLOWOUT, it will warn you to get off the freeway with the little cute light~

the battery in the TPMS sensor (sender inside the tire, part of the valve stem ) is probably run down ( 10 year life more or less) . If your tire pressure is correct per label, you need new sensors. Dont buy used (same battery problem) and only get "rebuilt" from a legit source (battery replacement was not intended / made possible by manufacturer originally) afaik, 4runners dont have TPMS reset buttons, a battery discoonet may do the trick,

unlikely your 4runner monitors the spare pressure, only the 4 corners on the vehicle Check it manually or its possibly useless as a spare