Your G4 iBook uses the 802.11b AirPort card, whereas the G4 PowerBook uses the 802.11b/g AirPort Extreme card. They support a maximum data transfer rate of 11 Mbps & 54 Mbps respectively.
The A1345 (4th Gen) AirPort Extreme does support both the 802.11b & g standards and both devices should be able to connect at their maximum data rate.
The AirPort Extreme's default settings would allow connections up to 130 Mbps if your G4 computers were capable of connecting at those speeds, but they are not.
In very simple terms, your AirPort Extreme can go 130 miles per hour on the 2.4 GHz band.
The G4 iBook has a top speed of 11 mph and the G4 powerbook will go 54 mph. There are no settings that will make the G4s go faster.
If you connected the G4s to the AirPort Extreme using an Ethernet cable, you could expect connection speeds of around 100 on the G4s.
A new computer connecting to the 5 GHz band on the AirPort Extreme could go 250-270 mph quite easily. The 5th Generation AirPort Extreme will support wireless speeds up to about 450.
If you don't have any "n" devices, one option would be to set the 2.4 GHz radio mode to the "802.11b/g compatible" setting.
The other issue which you won't be able to work around is wireless security. Unfortunately, your 802.11b AirPort card can only support WEP. This type of security is by today's standard unsecure and very ineffecient. Your only option to improve bandwidth would be to disable wireless security, but that, of course, would not be a recommended one.
On your Mac using 10.5.8......
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Extreme, then click Manual Setup
Click the Wireless tab below the row of icons
Click the Radio Mode settings box, which probably now reads Automatic
Select the 802.11 a/n --- 802.11 b/g setting
Click Update at the lower right and allow a minute for the AirPort Extreme to restart
I would be surprised if you notice any difference with this setting, but you never know until you try. Hoping I am surprised.
Please post on your results.