Giving an old system new life by installing more memory or a new storage device is one thing. But plugging a modern solid-state drive into an archaic 20-year-old Power Mac G4 requires a lot of hardware and software hacking, with the end result being questionably useful at best. Because clearly, a modern PCIe NVMe SSD that delivers gigabytes of throughput per second isn't going to deliver its best while running over a PCI bus with 133 MBps of bandwidth.
Apple originally released its PowerMac G4 in 1999 and then launched its upgraded iterations till 2002. In other words, this desktop is outdated, to put it mildly. In addition to the ancient CPU and GPU, it lacks almost all modern interfaces. So it's nearly impossible to use any modern component with it without complications. But that didn't deter Pierre Dandumont from Journaldulapin.com.
So what does Apple's early 2000s Power Mac G4 have to work with in terms of an interface? PCI. Not PCI Express, mind you, but a PCI slot that can be used to install various add-in-cards, such as graphics boards (yes, since the system is based on a Power processor, it lacks AGP as well). Meanwhile, there are PCI-to-PCIe adapter boards that let you install PCIe hardware into PCI slots, though with some caveats.
Just plugging things in is not enough to make PCIe hardware work with a Power Mac G4. That system for obvious reasons does not support the NVMe protocol used by modern speedy SSDs. But it appears to support the AHCI protocol used by modern hard drives, some PCIe SSDs, and all SATA SSDs. The Power Mac G4 shipped with SCSI or Parallel ATA HDDs, and it looks like Apple used PATA controllers that support AHCI, but we are speculating here. This AHCI support enables the use of PCIe SSDs that use this protocol, such as Samsung's SM951.
Yet even picking up the right SSD is not enough to make a relatively new drive work in a Power Mac G4 PC that's roughly 20 years old. In a bid to make the drive bootable, a different BIOS (or Open Firmware in this case) is needed and an appropriate version of Mac OS X is required, which Journaldulapin.com explains how to install.
After all that, there will still be performance limitations, such as the 133 MB/s bandwidth supported by a 32-bit 33 MHz PCI bus (keep in mind that we are talking about a half-duplex interface here), but this is to be expected.
A legitimate question about installing a new SSD into a system with such a performance-capping interface is whether it would be far easier and faster to just plug in an external SSD using a USB connection. And it is certainly easier to plug in a USB drive, but Apple's Power Mac G4 only features two USB 1.1 ports, which means a 12 Mb/s data rate (1.5 MB/s), an order of magnitude slower compared to what a 32-bit 33 MHz PCI interface provides.
Of course, using a FireWire 400 (around 50 MB/s) connection would have been faster, but connecting a modern SSD over a FireWire interface to a 20-year-old PC would probably have been even trickier than using a PCI slot.
Then again much, like many of the best Raspberry Pi projects, installing a new component in an outdated Power Mac G4 is a lot less about convenience or improved performance as it is about fun and figuring out what's possible. From that perspective, this endeavor was clearly a success.