I have no problems loading my MP4's into the Arranger of Samplitude Pro X3 Suite. However, the MP4 format is a container that can contain video that is not compatible with Samplitude or even video editors.
What is the source of your video files? Camera (which make and model), cellphone, GoPro, video capture, etc?
Can you post the output from MediaInfo?
FYI, the manual states that, "If possible, use only videos that work exclusively with keyframes and without interframes (DiVX or MPEG are not very suitable). If necessary, you can convert files into other formats and resolutions using the video editing program "Movie Edit Pro". We recommend the DV and MXV formats."
The best for smooth playback is MXV format, proprietary to Magix, that can be exported from Movie Edit Pro or Video Pro X.