Hello. I want to download the 4k file. Normally i download over the browser and it works just fine however, from past experience, there exists a five hour timeout. I.e. if your download doesnt complete within five hours, the download cancels. Considering the file is 20gb, this is borderline for me and i fear the download will abort near the end. For anyone who has managed to download it, what method did you use. Also can someone generate a checksum for the file please. Thanks everyone

I use internet download manager. (You may use Eagle Get or JDownloader)
After timeout Right click -> "Refresh download address".
A popup will appear.
Click "OK"
Go to download page and it will refresh the download.
Click on "Resume".
Now it will start from where it was left.

IDM is available as 30 days trial or you may use pre-patched version.

As amrit says, use a download manager. I use DownthemAll. But use whichever works for you.

I have downthemall but after a gog download times out, i can't resume

I use
wget -c --load-cookies (your cookies file here)

sha256sum:
0a419e7cf971279602ef1562b84fc042166881cb79b1598239cc0beb89fcd17f video_game_show_the_witcher_3_wild_hunt_concert_4K.mp4
sha512sum:
cb533bc74771cff10266b6874034b749ab16a0c8fc39e8ab47f9af935065447e669b648405ff8b7bb7ac19c590ed7d35a285611b305988 12cfcb5e182b948e27 video_game_show_the_witcher_3_wild_hunt_concert_4K.mp4

I have downloaded the file over https (just in Firefox) so it is very unlikely that my file is corrupted (it also plays fine).

File checks out thanks. Its strange because gog times out after five hours and Firefox says it has failed and gives you the option to retry the download. However pressing retry allows you to retry without having to first login to your account (which most websites require) . Not only that but the download resumes where it left off. Very strange circumstances

GOG's ADN uses a session ID that is IP specific for the download. Said session is alive for a few minutes before you need to get a new one, but if there is activity through it (meaning you are downloading through that session ID), the time doesn't expire.
When your download drops at the 5 hour, the session ID your browser has is still valid, thus pressing retry soon-ish allows it to reconnect and continue. That most likely means your browser is cutting the connection, not GOG's ADN.

I'm using clip converter and HD video converter factory pro to download videos. The first one is an online downloader, I usually take it to download short videos. When it comes to movies or long videos, I take the second one. Maybe you can have a check.