Well my point is (assuming for a moment that mIRC doesn't simply use WM and instead needed to include the decoding libraries) would you want an increase in size of mIRC by a considerable amount for the ability to play... all 4 .mp4 files? Plus from what I've read about mp4, most manufacturers are only seeing the potential for MPEG-4 as a video format, not for music. Everything I've read basically says expect to see mp3s for many many years to come.
But seeing as how mIRC does use WM, that means WM must support mpeg-4, which at the moment it doesn't. And as the poster said, Windows Longhorn will add support, but Longhorn doesn't exist yet.
