Be careful using YDL 4.0.1 on a new G4 iMac, on the 12" 1.33GHz model the fan does not come on when the CPU runs hot. I rebuilt the kernel (an intensive operation) and the whole laptop became very hot without the fan coming on. Probably not very good for it.
Solution: download the latest kernel source from www.kernel.org (I have 2.6.14.3) and configure in the following when doing make menuconfig:
Device Drivers->Macintosh device drivers->Support for thermal mgmnt on laptops with ADT746x chipset
Device Drivers->USB Support->Apple USB touchpad support
Device Drivers->Network device support->Ethernet (10 or 100mbit)->Sun Cassini support
The first sorts out the fan problem, the second enables the laptop trackpad (which doesn't work with YDL 4.0.1 by default) and the third sorts out a problem with the ethernet not working with this kernel.
The built-in speakers still don't work properly with YDL, not sure of the reason.