There isn't a universally agreed upon "fourth stage" of a star's life. Stars go through different stages depending on their mass. Here's a breakdown of how we might approach your question:

For a star like our Sun:

1. Protostar: A cloud of gas and dust collapses under gravity, forming a hot, dense core.

2. Main Sequence: The star fuses hydrogen into helium in its core, producing energy and shining. This is the longest stage of a star's life.

3. Red Giant: The core runs out of hydrogen, causing the star to expand and cool, becoming redder.

4. Planetary Nebula: The outer layers of the star are ejected, leaving behind a hot, dense core.

5. White Dwarf: The leftover core slowly cools over billions of years.

For massive stars (much larger than the Sun):

1. Protostar: Same as above.

2. Main Sequence: Fusion of heavier elements (like carbon, oxygen, and silicon) occurs in the core.

3. Supergiant: Similar to a red giant, but much larger and brighter.

4. Supernova: The star explodes violently, releasing a tremendous amount of energy.

5. Neutron Star: A small, extremely dense remnant of the core.

6. Black Hole: If the star is massive enough, it collapses into a black hole.

So, the "fourth stage" depends on the star's mass:

* Sun-like star: Red giant

* Massive star: Supergiant or Supernova

Let me know if you want to explore a specific stage in more detail!