This is a common problem faced by many people from around the world. When you try to update your macOS to Big Sur 11.4 it gets stuck during download and doesn’t progress beyond that.
This is strange problem because I’ve never faced similar problem before with any other macOS version.
Cause of the problem
Some people have suggested that it’s the size of Big Sur which is causing problem but I don’t agree with them. It doesn’t matter how big a file is, it shouldn’t get stuck during download.
I blame it to the Apple server.
Well there is no solution at the moment which works 100%. All you can do is to keep trying.
If I find solution then I’ll update this section.