"Branch" means to take a file that is shared between two or more projects
and turn it into a unique copy that can be changed independently, so it will
only work if you have a file selected that is currently shared.  It is
difficult to tell, but it sounds like you have two unique files in the 4.01
and 5.00 branches, and you want a non-shared copy of it in the 4.10 branch
also?  If so just drag the version of the file you want into the 4.10 branch
(creating a shared file), and then click on the file and "Branch".

Hope that helps.  If not describe the layout of the projects and files and
any sharing or pinning more precisely and I'll try again :^)

--Mark