Yes you can do this, the steps are below, my screenshots use the portable version of VLC.
- Right click on the mp4 file, and choose open with.
- Uncheck the box for "Always use the selected program to open this kind of file", and then browse.
- In the browse dialogue navigate to your portable application's exe file, click on it, and choose open.
- In the open with dialogue click on the newly listed protable application, and then click OK. The file will open in that program, and start playing.
Since you did not check the box for always using that program you cannot double click an mp4 to open it in that program, you will always have to use the open with menu, but that portable application will now be listed in your open with menu so the process will be easier to re-peat; just right-click, open with, and it should list your program right there as an option, choose it, and it will open the desired file in that program.