Have you checked your Kobo to see what version of the firmware it's running?
The MENU window will appear on the left - select 'SETTINGS'
In the 'OVERVIEW' section of the SETTINGS you will see the firmware version listed under DEVICE SETTINGS.
If the upgrade completed properly then it should say that the version is 1.4
If it doesn't say 1.4 then that may be the reason that you keep getting prompted to upgrade.