You will need to encode the .aac to .mp4 format.
Look at FFmpeg or VLC, they should be able to make the conversion. There are other encoders to choose from as well.
Another thing, is "test4.aac" located in the [install-dir]/content folder? If so your URL should look like this:
If the file is located outside of the content root folder ([install-dir]/content), you must add the default application instance name to the playback URL.