Two ways that I know of for a G3/G4
Mac2TV or ATI's USB TV tuner. Both I've seen at local CompUSA (Mac2TV) or MicroCenter (ATI's). Both are $99 and use USB to hook to your G4. They have a coaxial cable connector on one end, and software to change channels and resize the window. I don't know how big your screen res has to be to run full screen. My PM6400 has the Apple TV tuner card I added, but it only runs through the built in video port (1MB VRAM), and only runs full screen @ 640x480. Stinks, as I have a nice 8MB ATI card in the PCI slot, and almost never use the built-in video port for graphics work.

The Apple tuner will not send the signal to the PCI card, so I have to switch the cable constantly and reboot if I want to watch TV (I need a second monitor!). Oh, you'll need a third party PCI/USB card for older ADB serial port equipped beige G3's too.
Good luck.
Brian