iOS devices will not be able to view MP4 files that are attached in the forums, they need to be downloaded and played in an App that supports mp4 media.

If such files are attached a reminder to visitors may prevent posts about the file not playing on iOS devices.
Background from Apple :servers hosting media files for iOS must support byte-range requests, which iOS uses to perform random access in media playback. For full details, if interested, see https://developer.apple.com/library/content/documentation/AppleApplications/Reference/SafariWebContent/CreatingVideoforSafarioniPhone/CreatingVideoforSafarioniPhone.html#//apple_ref/doc/uid/TP40006514-SW6