MacOS – install Yosemite on a MacbookPro4,1 that has a broken firewire system

I would respond with this as a comment but I don't have that option, assuming because my reputation hasn't yet reached 50.

To me this sounds like a couple of possibilities. I don't see mention of whether or not the screen gets backlight, only that "it doesn't turn on", so I'm writing this assuming that the computer chimes but the screen remains black. It's good that you've tried reseating the RAM however I would also try to isolate both DIMMs. Remove one of the RAM DIMMs, try powering on the Mac. Reinstall the removed DIMM and remove the other DIMM, try powering on the Mac. Try installing the DIMM from slot 1 into slot 2 and vice versa. See if there is any change in behavior after each of these changes. If the issue persists with each change then RAM is unlikely to be the culprit.

Assuming RAM isolation has no effect I would try resetting the PMU. You didn't mention the exact model of your PowerBook G4 but this Apple KB article should help you identify the correct process for your computer. PMU resets typically have results when the Mac simply isn't power on at all however I've seen them resolve "no video" issues as well in the past.

If both of these had no effect I would start looking at it as a hardware failure, with either the logic board or LCD/display assembly to blame. You did mention that an external display had no video as well so I would lean towards a logic board failure. I would find it odd that the failure occurred after reseating the RAM but it could be a coincidence.

Also, PowerPC based Macs (G3, G4, G5) typically will not boot from USB devices. There ARE some methods for getting some specific models to do so however they often come with mostly mixed results. Your best bet for installing 10.5 would be doing so from a firewire drive or from the install DVD.

Hope that helps! Let us know if you have any luck with bringing it back to life!

I'd do a quick triage to boot the Mac into single user mode to see if the boot drive / base OS is minimally functional. After that, I'd start with systematically working through the startup troubleshooting guide from Apple.

• OS X: Gray screen appears during startup - http://support.apple.com/kb/TS2570

You've clearly done many of the steps, but the order of steps is very helpful in knowing what the likely cause of the problem is. I might skip removing the SSD if you don't have a HDD - but installing a clean OS onto an external HD at that point might be a good way to rule in or out your SSD as contributing to the failure to install and boot properly.