The Galilean satellites, in order going out, Io, Europa, Ganymede and Callisto, are of magnitudes 5.0, 5.3, 4.6 and 5.6 at opposition, respectively, so they would all be visible to the naked eye at Jupiter's present distance were it not for their close proximity to the glare of the primary. However, at under two seconds of arc in diameter, none would show a disc.

There is a report that one eagle-eyed medieval Arab astronomer supposedly described them accurately. It is not an unreasonable claim. The average human eye can resolve down to about a minute of arc, and the planet's disc (142,980 km in diam) at opposition is about 50 seconds of arc, right at the theoretical limit of human visual acuity.
Io is 421,800 km from Jupiter, Callisto is 1,883,000 km, so it is within the theoretical capability of a sharp human eye to resolve the system.

In practice, of course, the disparity in brightness between the primary and its satellites tends to blur them all into one blob of light, which is easily resolved into all five components with only the slightest optical aid. Maybe God played a joke on us by putting the system just far enough away so that we would have to wait for Galileo and his telescope.

The next time the moon occults Jupiter, perhaps one of us could use our moon to cover the Jovian disc and perhaps reveal the four moons to the naked eye. Does anyone out there know if anyone has done this?
It would probably take just the right conditions to see them all, but at least one should be visible.