Hello ! > I also can't use more than 1,6G of RAM, although there's more in the machine.
That is normal (I have here 2 DP G5s) because OpenBSD runs only in 32 bit mode (even the G5 is a 64 bit CPU). See https://www.openbsd.org/macppc.html section History. The max RAM in the G4 (G3 less) was 2 GB see https://everymac.com/systems/apple/powermac_g4/specs/powermac_g4_450.html as example.
