Unfortunately there is no one standard format for MP4. Generally installing the K-lite pack will allow you to use MP4 videos with popVideo but if the videos you have use a specific codec that even K-lite doesn't support, then sadly you may not be able to get them to run.
Try using a codec identifier like
GSpot to check your videos. This should tell you which video and audio codecs are being used and identify any problems.
Peter
Forum Administrator
www.reallusion.com