As said
here, your PowerBook is at the bottom of the specs required to cut HDV footage. Also the camera records a MPEG-2 stream to the tape, iMovie will convert it to AIC (Apple Intermediate Codec) for better editing. The data rate will be around 100 Mbps (12.5 MByte/s), so you need a fast hard external FireWire hard drive, and a good processor.
I once used to play HDV footage on a Core2Duo iMac with 2GHz, and the CPU was up to 50-70% usage.
So don't expect any wonders with your G4.
A white MacBook would do better.