Big Sur 11.4 download hangs with "About 15 minutes" remaining.

You might try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - go to the App Store, search for and download the full Big Sur installer.

#3 - run the installer from the Safe Mode.

I've had better luck goring thru the App store than thru the System/Software Update preference pane.

This method may seem unnecessary but sometimes the Router Restart clears Network issues from ISP to Apple Servers

Shutdown Router. Shutdown computer and disconnect all External Drives. Restart Router. Connect to Router via Ethernet Cable. If via Wifi - Disconnect all other Devices using the Wifi - want the Maximum Bandwidth only for this computer. Restart computer in  Safe Mode. It does a Repair Disk, Clear System Cache files and only load Apple Software, extensions and Fonts. Will load slowly - Normal. Once Logged in - open Apple Apps Store and  Get the Full Version of Big Sur. It will take some time and several pauses will occur - normal. Countdown clock may say 15 minutes but in Real time may be 30 minutes. Same goes for clock at 10 minutes could be 20 minutes in Real time. Eventually , Full Installer will download and a New Application in Applications Folder will present called Install Big Sur. Stop there and perform a Time Machine Backup. If desired - have an External Drive - Copy & Paste the Install Big Sur Application to external drive for future usage - making a Bootable Installer. Now run the Installer.

Have found this method more reliable than the Software Update which is very iffy at best

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation.

If it is happening with direct ethernet and you don't have proxies, or System Preferences -> Profiles, or file vault interfering, or VPN, and no other software is running, and you've tried to reload it after deleting all components in the Applications folder,

then what the problem is, is probably the traffic on Apple's servers.

Being it is only 3 days since release, that's entirely possible.

But this is something you really need to also report to AppleCare directly.

and submit feedback to them http://www.apple.com/feedback/

Prior to April 27th this year, updates came at least on the download server on the website for Catalina and earlier, but Big Sur began the software update tool only method of running software updates. Unfortunately this has put all the eggs in one basket.

Ah - the old AntiVirus and VPN issue again interfering with the Normal Operation of OS. Great is was fixed by getting rid of this software