4 years old is not too old for a pacifier as I had my pacifier until 8 years old and then I eventually gave it up.
I stopped using the pacifier during the day when away from home at age 5 but kept using the pacifier at home until 8 years old and I was okay.
It never harmed my teeth in anyway.
Now that I'm an adult I went back to my pacifier use and suck on a pacifier every night when I sleep and during the day when I'm not at work and at home.
The pacifier helps me to relieve stress and they do make pacifiers for adults for medical reasons too so they're not just for toddlers or babies and adults can benefit from pacifier use as well.