I had problems upgrading to 1.4 and the issue was simple to resolve and I could not find this in the docs, so hopefully this will be helpful to someone else.
Follow the normal in place upgrade steps then navigate directly to <yoursite>/Admin (do not just go to the homepage because that will probably fail).
Open Modules on the Admin menu and then enable the following:
Title module in Core
Fields module in Fields
Enable the UpgradeTo14 module in Uncategorized
Title is the one I missed the first few times, I'm not sure why it is not enabled automatically when you enable UpgradeTo14 like Autoroute, but it isn't. Core developers perhaps you can comment on this?
Now open Migrate to 1.4 on the Admin menu.
Check all the types.
You should get a confirmation that all your content was migrated successfully.
I did not have any fields to migrate so I can't say for sure if there are any issues to watch out for, but I assume you should check the Migrate Fields tab and migrate those too if you have any.
You can go Reports on the Admin menu and click the link to UpgradeTo14 - you should see no errors, just a list of the successfully upgraded types.
Go to your site!