Yes, you are right, I also noticed that some files that BSplayer can render without problems do fail in GSpot.. Anyway MP4 files can have AC3 audio streams in them, so (if this is the situation) he will need AC3Filter to play it.
3ivX is a good codec to handle AAC audio streams, I only wanted to integrate my suggestion to your.
I had problems once with an ACM encoded file that GSpot recognised and let me install the correct codec to play it
