If you’re having an issue with MP4 videos not playing on mobile devices, and you’ve already added a mime type for MP4 in IIS, then it could be due to the “AllowedInlineDownloadedMimeTypes” setting of your web app.

To fix it, you can update the setting in PowerShell, with script such as:

$webApplication = Get-SPWebApplication “http://intranet.contoso.com”
$webApplication.AllowedInlineDownloadedMimeTypes.Add(“video/mp4”)
$webApplication.Update()