QuickTime is software that comes on all Apple computers and allows you to play video and audio files you have downloaded onto your computer as well as to stream content from the Web. Modern versions of QuickTime support mp4 files natively. Mp4 files, or mpeg-4, are video files created by the Moving Picture Experts Group that use different compression for audio than they do for video, according to the website File Info.
Open the “Applications” folder and click on “QuickTime.”
Click “Open File” from the "File" menu.
Click the mp4 file you want to watch in QuickTime and click “Open.” Click “Play” to watch the video file.