Implement 4-Digit MPIN for Android App

I need to implement a simple 4-digit MPIN for my existing Android app.

- The MPIN should be set during account registration.
- Basic numeric security is enough for this project.
- Additionally, I would like a recovery option through a mobile number.

Ideal Skills:
- Experienced Android developer with a solid understanding of security principles
- Previous experience with implementing authentication systems
- Knowledge of mobile number recovery systems would be a plus.

Please ensure that you have the necessary skill set to complete this task before bidding.