I literally just want to make a simple wii program that can play an mp4 or some sort of video container, is there any library that can achieve this?

Playing a video is not a simple task.

Doesent really help for you to say that. I'd really like to know how, no matter how challenging it may be.

First you need a demultiplexer to get the raw audio and video streams from the container.
Then you need a decoder for the video stream.
Then you need a decoder for the audio stream.
Then you need a video renderer to display the decoded video.
Then you need an audio renderer to play the decoded audio.
Then you need an actual player engine that controls all the other parts and keeps everything synchronized.

There can be no such thing as a "simple program" that can play an mp4, because playing an mp4 is not a simple task.