There is no two disc set for OS X 9.1. Such a beast doesn't exist. Apple has Mac OS X 10.9.1 which only came as an online download.
But it also has Mac OS 9.1 (no X indication) that came on a white disc with an orange 9 logo, and some gray discs with a Mac model name on it,
and some of the orange 9 logo discs have Update labelled on them. The Powerbook may be able to use the 9.1 disc, if it shipped with 9.0.4, 9.0.2, 9.0 or 8.6.1 or earlier:
PowerBook G4 Computers: How to Identify Different Models
Mac OS: Versions, builds included with PowerPC Macs (since 1998)
The gray 9.1 installer disc will only work on Powerbook G4s that came with 9.1, and the disc is labelled Powerbook G4 for that specific model.
10.4 will work on Powerbook G4s as long as they are older than the 10.4 installer disc, and the 10.4 installer disc is a retail version as identified on this tip:
https://discussions.apple.com/docs/DOC-2541
Or the 10.4 installer disc has Powerbook G4 indicated on it, and is for the same model of Powerbook G4 you already have.
Is there some reason you want 10.4 on your machine? If it is to run Classic, you'll need an external hard drive to backup your data, before installing Mac OS 9.
If it is to boot Mac OS 9, and your Mac is capable of it:
Mac models capable of dual-booting into 10.5 and Mac OS 9
You'll still need to backup your data before reformatting it to have Mac OS 9 drivers.
Powerbook G4's older than 9.2.1's release may use the 9.2.1, 9.1, 9.0.4, 9.0.2, or 9.0 retail depending on if they are older than that retail release.
Powerbook G4s older than july 1, 2002, but newer than 9.2.1's release must use the prebundled Mac OS 9.2.1 or 9.2.2 gray disc to install Mac OS 9.
These older Powerbooks to optimize their use of Classic will need 9.2.2:
How to find Mac OS 9 downloads?
Powerbook G4s newer than July 1, 2002 must use their prebundled restore discs to install Mac OS 9.
Edit: Update discs will only work when booted from an older version of the same system.