Is it possible that (a) you have no monitor connected to any of the
systems' video outputs and (b) the SS4 has a framebuffer present and
keyboard connected and the 10 and 20 are lacking either the framebuffer
or the keyboard? That could account for the '4 defaulting to video
console with the '10 and '20 defaulting to serial console.
Or perhaps that the '10 and '20 have input-device and output-device set
differently from the '4?
The system boots and I can telnet into it!
What does eeprom(8) print for input-device and output-device on the '4,
as compared to the '10 and '20? If you'd rather not bother booting
them, what do `printenv input-device' and `printenv output-device' at
the OBP ok prompt produce?