A more reasonable question would be, "How can I make my own bootable media?" The simple answer is you can't do it legally. The Open Firmware "second level bootloader," like the one written for U-Boot, is copyrighted, or at least not open source. (As an aside, I'm surprised. I've read that slb makes use of GPL code, but I don't know for sure.)
The good news is that if someone can reverse engineer the process and publish an open method for booting OS4 on Mac hardware, you're good to go in the US, as hardware for OS4 is no longer reasonably available. (Hey, the DMCA exemptions are good for at least some things.) You must, of course, own a copy of OS4. Now that the classic version is available, that shouldn't be an issue.