SPARCStation 4
I currently have a Sun SPARCStation 4 sitting in my room. Of course, the unique monitor connection makes it impossible for me to fire it up without an adapter. eBay should have one. Once I have one of those, I should be able to turn it on and see if it works and stuff.
You should be able to hook up a Serial Cable with a Null Modem adapter to port A on the sparc and fire it up.
Make sure the Sun Keyboard is unplugged from the Sparc when you do this , The Keyboard is what triggers the onboard video/keyboard instead of Serial.
You will be able to access both Open Firmware and whatever OS is installed on the Sparc if it has a OS present.
BTW, The Video Adapter you purchase may not let you use your Sparc 4 with all monitors, Sun uses non PC/Mac video resolutions and not all monitors can support them.