The ability to install, reinstall, or upgrade to Tiger remains important, but it is increasingly difficult. Mac OS X 10.4 installers have been discontinued by Apple and are no longer sold at retail. Finding an installer requires buying a new or used copy from a reseller - or borrowing a copy from a friend, consultant, or other Mac user.*
To increase the challenge, there are different versions of the Tiger installer for PowerPC and Intel Macs. Apple never released a Universal Binary of the full installer, instead releasing separate PowerPC and Intel versions. Some later updates were available as Universal Binaries, but not the full OS installers.
Every Mac sold during the Tiger era came bundled with a grey Mac OS X installer DVD. These discs will reinstall the OS on that model of Macintosh, but they may not work with other Macs using the same processor. There is (I'm sure) an underlying consistency to what discs will work with what models based on system architecture, but these relations are often not user discernible. On practical level, it's usually a trial-and-error process if you don't have the right disc.
On the PowerPC side, the situation is easier. Apple sold several versions of Tiger at retail, culminating with a 10.4.6 full install DVD. These discs will work on all supported models of PowerPC Macs and are the best ones to have or try to find via resellers. The bundled install discs that came with G4 and G5 Macs do not work across the board on all models - again trial-and-error.
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All Intel Macs came with their own (grey colored) Tiger install DVDs. Apple never sold a Full Installer for Tiger that was Intel-compatible. No need - if you were buying an Intel Mac, you automatically got Tiger, and if you had a PowerPC Mac, you wouldn't need an Intel-compatible installer (nor could you use it).
This makes sense - until you need to reinstall Tiger on an Intel-based Mac. The original Intel iMac, MacBook, MacBook Pro, Mac Pro, and Mac mini models (which came with Tiger from the factory) all use different custom install DVDs. Most systems require their own discs, again hit-or-miss will apply.
I have had luck using the 10.4.10 installer that came with my Mac Pro to reinstall Tiger onto an early MacBook, but a MacBook Pro installer from the same era didn't work. You do need the original discs - just at the moment (of course) when you can't find those little envelopes that came bundled with the snazzy Apple cleaning cloth and window stickers you safely saved for future use.
Yes, you know they're around somewhere....