Well assuming the video was written to the card in the first place and was somehow deleted you can try file recovery software, there are a lot of different freeware software recovery tools available Recuva is one, it's supposed to be pretty good and easy to use.
The question being was the video actually written to the card? If not then there is nothing that can be done unfortunately, you can't recover data that doesn't exist.
I'm using RecoverX. It's been 3 hours it shows there is one more hour to go. Is it a good sign? Also I went to the folder Recovered Files there is a lot of files but I cant open them (yet, hopefully). But a lot of them weigh 256kb which is impossible that clips worth of 4 hours recording weighed 256kb. Maybe it'll change once the recovery process is done?
It's worth the time trying to use the free recovery stuff first. If it works, great. If not, you'll need to decide whether the file is worth spending money on.