The only model of G4 Tower that didn't have a DVD-rom was the very first release (PCI Graphics, 350 or 400 MHz G4 processor).
If that's your tower, then yeah, you're outta luck, unless you want to buy a dvd-rom drive for like $20 and install it. Other options, borrow an external dvd drive (or buy one and return it when you're done), or another friend's mac that can read DVD and connect your Mac to theirs via Target Disk Mode and install Tiger from there.
FYI: It wouldn't be a bad idea to maximize the RAM in your tower (1 GB max) to accomodate for the finely-aged processor. (processors are like wine, right? )