Disclaimer: I've used Linux and FreeBSD since 1999.
I wouldn't suggest installing Linux for the first time on an important machine. As kenmasters demonstrated, it's not entirely idiot-proof yet. It's a fantastic OS, inarguably on a rough level with Windows and OS X depending on your criteria, but it has a stiff learning curve unless you are lucky enough to have everything "just work" -- which might not be the case with a Mac install.
Once all that's done, you'll never need to do any significant maintenance. Its stability is untouchable (except perhaps by FreeBSD, which admittedly has been sagging of late), its functionality is mind-boggling, and the general open-source infoanarchism is a refreshing change from the corporatonomy of Apple's and Microsoft's apps. Linux media centers can be extremely sweet.
So it just depends on how important that G4 is to you right now.
On the subject of distributions, my personal suggestion is Gentoo. Gentoo's probably the best-maintained "power user" distribution out there. It's pretty hardcore for a non-programmer and very hardcore for a non-Linux user. However, I find it to be a great distribution for newbies because it involves building your own Linux system one little step at a time, and essentially learning about the structure of UNIX and Linux as you install. A full-blown distribution like Ubuntu or Yellow Dog can be overwhelming, especially since you as the user are immediately made aware that the "depth" of Linux is bottomless. With Gentoo you start at the bottom, with very few programs and choices available, and configure your system to run exactly and only what you need. The result is a very fast system that is tailored precisely to your needs. As an added bonus, the support forums are second to none in this universe.
Again, that's not something to use a working box for. But if you're curious in pursuing this project, drop $50 bucks on a cheap old Athlon PC or something. It'll take a few days to install Gentoo, and it'll be infuriating and hellish and exhilarating, and you'll come out of it with a good starting knowledge of the UNIX way of doing things. It'll not only be a fun waste of time and money, but it'll also deepen your knowledge of OS X internals and possibly make you overall a happier and more confident computer user.
Just something to think about.