An avseq file is a particular way of encoding video files. These files are named avseq.dat and are used when placing a video file into a VCD (virtual CD image). A .mp4 file is a video file using the MPEG-4 compression format. The avseq.dat file within the VCD can be converted to an .mp4 video using the appropriate software application.
Use Xilisoft Video Converter (free trial, see Resources). In addition to converting the video file, this program can edit the video by cutting out certain frames or scenes or by adding video effects.
Use Format Factory (free, see Resources). This program supports a variety of formats, including .dat and .mp4. It can additionally repair damaged files, reduce file size and supports subtitle files.
Use YASA Video Converter (free trial, see Resources). In addition to reading and converting .dat files to .mp4, the software has many options to customise the output file. You can change the video frame size, video frame rate and audio bit rate of the file, allowing easy customisation and control of file size.