What are the four stages of PSLV?

The four stages of PSLV are PS 1, PS 2, PS 3, and PS 4. PS 4 is the final stage and PS 1 is the first stage of PSLV. You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Chandrayaan 2 Mission
2. Mars Orbiter Mission (MOM) – Objectives, Payloads