Exported mp4 movie bad quality, deleted imovie project! Help!

Hello,

My girlfrien exported an iMovie project using Quicktime in mp4 format, but forgot to check the quality. Now the quality is extremely poor and this has happened to all her videos. She deleted the original projects, including the iMovie clips. All the movies were exported to an external HD and there is no sign of the original files in the computer.

Anyway you can compress/decompress the video back in order to gain good quality again?

Best