MPEG4 support was not always a feature of VDPAU. This was a feature added in 2009 and may not always be defined depending on distribution.

See: http://cgit.freedesktop.org/~aplattner/libvdpau/commit/include/vdpau/vdpau.h?id=46e791147ac43ecd2fbfbc6bdf3bebda9030b51e