Unpaid Salary

Respected Sir/Madam, I'm writing this to share my issue and seek your advice on how to deal with it. I worked with a private limited startup based out of Hyderabad between October 2017 to August 2018. I relocated from Mumbai to Hyderabad during that period. During that period, I spent on my travel, accommodation, and all other expenses out of my pocket. Over the period of time when I realized that the company is unable to pay me the promised salary, we mutually decided to part ways and I moved back to Mumbai. At the time of parting ways, the company issued me a letter of outstanding dues payable to me from the company. However, they consistently failed to pay me the promised salary month after month and till today the dues are unpaid even after 4 years. Please help me understand what are my options to recover my dues and the chances of recovery, what would be the process I'll have to follow? I highly appreciate your quick and positive response and thank you in advance for the same, Looking forward. Roshan