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.
If you have customization, there are improvements that make handing Javascript libraries easier to manage. From a developer’s point of view there are numerous reasons to switch, especially if you plan on adding more custom code.
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.