Trim mp4 video is quite problematic because not all programs support this function. Modern video editing software is very expensive and includes a lot of features. Free programs have limited functionality. I found the Best Software to Crop MP4 Files list for you (
https://www.movavi.com/learning-portal/mp4-cropper.html). One of the most common and free programs for Crop MP4 Files is Windows Movie Maker. However, if you like to edit videos entirely in one application, then it is better to choose software like Movavi Video Editor Plus.