Recently I have friends sending me messages with attachments named MP4 and I am unable to open/view what they are sending me. My question (although not super major) is there some type of setting that needs to be implemented in order to view that I don't have already established? I just don't don't what they are sending and I do know it is not spam or junk as they are my regular friends

As usual thanks for the time (and help)

Think you need to add an entry to viewer for the MP4 Extension.
Should be able to do it in tool / options / content viewer
I used the Mime type
Aded a line to VIEWER.PM
2;MPEG4-video-file;-;Z:\usr\bin\pegattach

In my case Z:\usr\bin\pegattach is a linux strip the extracts link and sends file path to gnome-open that uses the default linux program. You can probable just select to use the windows program.

Open the attachment view, and then clicked on first run option to view test message.

Maybe there are two preconditions to be met here: As can be seen in Mikes reply MP4 denotes video format files, i.e. for displaying such videos you would need to have some software installed on your system being both capable of playing video files and have it configured to be the default application for opening files with this file extension.

And depending on which Windows version you're using there's another issue: IIRC Windows 10 comes without DVD support and more importantly in this case without an accompanying video player software, so if you've never been watching videos outside of your browser (which most likely will be able to display embedded MP4 videos without any additional software required) you'll need to get and install such a software and enable it to play MP4 video formats by default.

Only then Mikes suggestion will succeed as well ...