I stand corrected, 4″ was the right choice. Always nice to emerge from the murky middle of “why the fuck did I think this was a good idea,” “how the hell did I ever expect this to work,” and “this whole mess is trash and I should scrap it.” If you can’t trust the process, be too obstinate to stop.
The reason is that the inner three Galilean satellites — Io, Europa, and Ganymede — are locked in what is called the “Laplace Resonance,” in which their orbital periods have a ratio of 4:2:1. This means that for every one orbit Ganymede takes, Europa makes two and Io completes four. Io’s volcanoes are, in fact, a direct result of this relationship because the gravity tugs of Europa and Ganymede, combined with the tidal force of Jupiter, produce a continuous source of heat for Io’s interior.