I have an infection in a tooth that has had a root canal done. I think ive had it for longer than 4 to 6 years. What will happen if left untreated?

Continued Infection. The bone will continue to break down, the infection will continue to spread, and you most likely will lose the tooth. Ignore your teeth and they'll go away.
Infected tooth. It is hard to believe that you could have an infected tooth for so many years without severe symptoms. An untreated infection of a tooth can lead to an abscess of the jaw which can cause severe pain and total body infection and even death. You need to see a dentist and have X-rays and if indicated antibiotics.