While .mp4 files themselves are generally safe, you should remain cautious of files from unknown sources:
: .mp4 is a standard digital multimedia container format most commonly used to store video and audio. Cloudmail (3) mp4
A file named is typically a video file downloaded from a web-based email service or a cloud storage provider. The "(3)" in the filename usually indicates that it is the fourth copy of that specific file downloaded to your device, as operating systems automatically rename duplicates to avoid overwriting existing files. File Characteristics Cloudmail (3) mp4