Most multi-episode files I’ve run across have chapter markers denoting where the next episode starts.
If you need to re-rip the discs, try MakeMKV. It rips an unaltered copy of the movie/show to your hard drive, saving it as a MKV file.
You can then use MKVToolNix to separate each file into the individual episodes. If chapter markers are not available you can use timestamps instead. See the support article linked below.
Note: MKVToolNix will take a mp4 file as an input. If you do not want to re-rip the discs, use your existing mp4 files and split them at the appropriate points. The output will be a MKV file. There’s probably a tool somewhere to split mp4 files, but I’m not aware of one.
After splitting the file, you can transcode it to another format, container, etc with Handbrake or similar tools if needed.
