There are actually three different models of 400MHz G4 tower--the original "Yikes" (PCI Graphics) model, the slightly newer AGP Graphics model, and a generation later the "Gigabit Ethernet" version. They all look nearly identical from the outside.
Fortunately for you, all three should have 16-bit analog stereo in via a standard line-level 3.5mm jack. The model afterward, Digital Audio, removed the audio input jack, but the slowest version is 466MHz, so that's not what you have.
If your source is RCA (red and white jacks), you'll need to get an RCA to 3.5mm adapter (usually sort of Y shaped), which can be found cheap at most electronics stores (as they're also used to output sound from an iPod or other 3.5mm stereo source to the RCA inputs on a stereo).
