PowerMac G4 Cube powered over USB-C!

USB-PD 3.1 supports up to 240W, so from a wattage perspective, you're fine. You'd probably want to ensure there's some sort of handshake/regulation to ensure it's not drawing more than 210W though. And it'd need to be USB-PD 3.1 or later to go that high.

Of course. More amperage is always OK

KMM said:

G4 Cube adapter has 28V 7.5A => 210W. If I use adapter that has V<28 and A<7.5, regardless of its power will drive my Cube or not, it will not harm my G4 Cube theoretically, Am I right?

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The adapter requests 28V 5A from the PD charger. If the Cube tries to use more than 5A then something might happen. The adapter or charger might detect overcurrent and stop providing power or something in the adapter or PD charger (but not the Cube) might get hot. If something gets too hot then it might melt.

The original post says the Cube usually doesn't use more than 4A as long as you're not trying to power an ADC display.

Joopmac said:

Of course. More amperage is always OK

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On the USB side, amperage/current (A) is limited to 5A. Increased power (W) comes from increasing voltage (V), up to 48V.
The amperage is not increased since that requires thicker wires.

It doesn't matter what PD charger you use, since both sides will negotiate a voltage/current that is compatible to both sides. The adapter's Cube side requests 28V/5A. If the PD charger side does not support 28V/5A then there shouldn't be any power supplied to the Cube side (voltage will be too low/zero).

Thank you very much for all helps/suggestions. @adespoton @Joopmac and @joevt .