How do I watch a 4GB film in 'disc image file' format on my computer? I tried VLC but only one "extras" clip is working but without any audio and when I click on any other options on the home screen nothing happens. I thought VLC played everything.
It really depends on what this
VLC has some success opening some image files directly, but, I have known it to fail sometimes for unknown reasons.
Why it is failing in your case - I can't honestly say without a lot more information.
Providing this is a standard image file, I would recommend downloading Virtual Clone Drive then mounting the disk through that, so, it appears to the computer that you have inserted that disk for real - then relaunch VLC choosing the (virtual) drive instead of the file.
If it works - great, it was one of the many VLC bugs that seems to be happening lately. If it still doesn't work - it could simply be that a codec is missing from your machine that is required to play it.
Depending on what codec it is, you may have some luck installing the K-Lite Codec pack.
I would think it is more a case of codec missing - especially if the disk is high definition or has some similar special features (3d etc.).