I found it easier to log into the router via SSH as the root user and edit the config, but I'm sure there's a way to do it through LuCI. You can edit /etc/config/network. For me I added option ipv6 '0' (with a tab) below config interface 'wan'.
You may have other wan networks such as wwan if you're using your router as a repeater or range extender. Under LuCI's Network > Interfaces, if you see any others that don't have any errors, add that same option ipv6 '0' with a tab below each config interface '<interface name>'. (Opkg update error (wget returned 4) - #8 by komawoyo)
For me, under LuCI's Network > Interfaces, the only other wan is wan6 and it shows the error Error: Network device is not present so I decided not to mess with that.
After I found all the wan interfaces and added those, I ran /etc/init.d/network reload. That helped make opkg work better for me but it might work for your issue with wget. (Opkg update error (wget returned 4) - #9 by helpme555)