How to recover an iMovie
The short answer is you recover this file in the same way you'd recover any other file, but this assumes you're only wanting to recover the final movie you produced and not all of the raw footage and project files? It also assumes you exported the video in a playable format (i.e. not just to DVD, but also as a single video file such as a .mov or .mp4 file)?

If so, any good data recovery package will provide filtering options that make it easier for you to isolate and recover what you want. In your case, if you know you saved it as an .mp4 for example, and that the file size was about 4GB, you should be able to select .mp4 as a file type and select any matching files within the vicinity of 4GB in size.

If you're not 100% sure of the file type, any good data recovery package will also group files into categories such as Pictures, Movies, etc. In this case you could select any video or movie files that are around the 4GB size.

Another option some packages will give you is the ability to see the original path of the files. If so you can look for files in your user's Movies folder in which there should be a number of iMovie folders and files. This may allow you to browse and find what you're looking for.

Did you create a disk image instead?

Many standard definition videos of about 1hr duration compress to about 4GB in size when saved as a disk image, so it is possible that you've done just that. Of course, if it was a high definition video then it's duration would be a lot less if it resulted in a 4GB file. 

Regardless, the fact is iMovie provides the option of saving it's projects as disk images. In this case you would be looking for a .img file to recover. These files do not play in Quicktime by default as they're not movie files, they're disk images. 

You will need to mount the disk image first before you can access the data. Typically you can just double-click on them and this will mount the disk just like any other drive (i.e. it will appear just like you plugged a USB stick into your computer).

Once the disk image is mounted, you can access the files. If what you see is a movie file you should be able to play it in QuickTime. However, if what you find is two folders labelled AUDIO_TS and VIDEO_TS then what you have is a DVD disk image. You can play this in the DVD Player app, or you can choose to create a physical DVD from it using an app such as Burn or Toast to copy the disk image to DVD.

NOTE

You should be aware that recovering data is not perfect. You may be able to recover the file and all is good, or you may find that it's also corrupted in some way (e.g. the video data is there, but the audio data is missing, etc). Also, it may be that the data isn't all located in a single file any more, or that some of it is missing, so if you have no luck finding it by recovering files around the 4GB size, you may need to recover files well below that in the hope of some success.