My teeth are moving back to old positions though i've been wearing retainers for 4 years after wearing non-removable braces for 1.5 years.

Need re-evaluated: Return to your dentist and have them do an evaluation to determine the cause of your relapse.

Relapse: Important that you see an Orthodontic Specialist to determine why your teeth are unstable. Make sure you see a specialist, not all Dentists who do braces treatment are Orthodontic Specialists. If the Dentist that provided your braces treatment is a specialist, that's the best person to see to address your current dilemma.