Thanks for asking the query,
As you have mentioned your four teeth are decayed, I would suggest you to visit the dentist
and get the checkup done, take an x-ray of the tooth.
Since you have mentioned there is no pain in the tooth the tooth can be treated with restoration.
If the infection is deep involving the pulp tissue can be treated with root canal treatment
can be due to extrinsic stains such as plague
and calculus deposition,consumption of coffee,tea you can go for symptomatic treatment of complete mouth scaling and polishing.
If the yellow discolouration is due to intrinsic stains can be because of hereditary conditions, excessive fluoride
intake this can be treated with bleaching, veneers and bondings.
Hope this helps out.