There is a significant improvement to the interface and the use of tabs enhances the user experience in the 5.x versions. There are several bugs that 4.1.x has that were fixed in versions 4.2, even more in 5.0.
Also, I have done custom upgrades from 4.x to 5.0 and 5.1 for 8 or 9 customers. Never have I heard once that they wished they could go back. For people who are resistant to change, they can keep the old 4.X frames UI. For those who like the tabs view, they can choose to personalize their experience using tabs and it doesn’t affect anyone else.
If you need assistance with bringing in your custom changes to 5.0 or 5.1, I’d be happy to consult.