Help me with quicktime errors?

I downloaded a .mp4 video file.

When I tried to open it it said 'Error 2048: Couldn't open the file because it is not a file that QuickTime understands.'

I'm probably doing something wrong..Is it in the wrong file format or something?

Help please and thanks!

What version of Quicktime are you using? It could be the version you're using is an old one. Try downloading the latest version, and then try opening up your mp4 file again.