what are these four moons collectively known as?
The Galilean moons
Galilean moons
The Galilean moons (or Galilean satellites) are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth.

io and europa the inner two of jupiter's four largest moons They include also Ganymede, and Callisto
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Galilean moons [SEP] Introduction
The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's much smaller remaining moons have irregular forms because of their weaker self-gravitation. The Galilean moons were observed in either 1609 or 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than ever. Galileo's observations showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted Ptolemaic world system, a geocentric theory in which everything orbits around Earth.