Galileo didn't *only* see four moons. He was the first person to observe and document the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. These are now known as the Galilean Moons.

Here's why he didn't see more:

* Telescope limitations: Galileo's telescopes were relatively primitive compared to modern instruments. They had lower magnification and were less precise. This meant he couldn't see fainter or smaller objects.

* Small size of other moons: Jupiter has many more moons than just the four Galileo discovered. However, most of these moons are significantly smaller and fainter. They wouldn't have been visible with Galileo's telescope.

* Observational challenges: Even with the best telescopes of his time, observing these moons was difficult. They were close to Jupiter, and the planet's brightness could make the moons hard to see.

It's important to note that Galileo's discovery was still a groundbreaking achievement. He was the first to observe objects orbiting another planet, providing strong evidence against the Earth-centered model of the universe. His discovery helped pave the way for future astronomical discoveries.