If you don't have a zip drive you can also shove it into the zip drive bay.
IMHO I would NOT stick drives in the U carriage like that. If you can, grab yourself some longer ATA cables and stick them on the bottom, same for the 3rd drive. You can have all 3 next to each other this way (1 in front, 1 in middle, 1 in back), and still take advantage of good ventillation.
Like I said, you'll need long (30" maybe) ATA cables and it won't look as pretty as the ones apple has (unless you go for rounded cables), but you'll be much more satisfied with the result (the drives won't overheat, they're easier to swap when in lower bays, etc).
IF anything, you can take out the drive trays and then when you put them back in use thumbscrews for even easier access
As said, you can have up to 4 ATA devices on your g4 without another motherboard. You can set one to be on the same chain as the CD drive but it may or may not run slower (I think it's an ATA33 bus).
Otherwise you can also buy an ATA100 or 133 (or even 150) card and try that.. but I still highly reccomend this mounting strategy when using quicksilvers for multiple drive slike that
If anything, you can probably find on a used parts site (if you raelly wanted to load it up with drives), a second dual drive carrier and stick it in the front (the middle won't work due to cables on the motherboard)... But I still say stacking drives like that can definately cause heat probs.
Hope that helps
