Its not a problem. The G4 CPU was a current chip around 4 years ago, and hasn't been significantly updated since. High Def footage needs an incredible amount of processing power to play properly. For example, if I disable one core on my 1.83 Ghz MacBook, resulting in essentially a 1.83 Ghz Pentium M, then it would not be able to play 1080 footage, and may choke on small bits of 720 footage.
The problem is that the CPU is just way too underpowered in pre-intel Apple notebooks