Suggest checking in Applications folder for an Application called Install Big Sur and Drag and Drop it to Trash and empty Trash.
There are factors like poor Wifi connection, slow wifi connection, too many other people downloading at the same time and Apple Severs are overloaded, Software on the computer interfering with the download process, running behind a VPN and or / if via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to you computer.
Best Results are via Ethernet Cable to Router
There are several ways to download the Big Sur 11.4.
1 - Connect to Router via Ethernet connection and restart the computer in Safe Mode and then open the Apple Apps Store and get the Full Installer of Big Sur. It will be the 11.4 version and once downloaded run the installer from the Applications folder.
2 - Resart the computer in Recovery Mode ( INTEL Command + r ) and choose Reinstall macOS. It too will download the full Installer and commence the installation. For M1 CPU the Recovery Mode is Here