You can have both v3 and v4 at the same time. However, files saved in v4 can’t be opened in v3.
The update is automatic. Back up all your working files before upgrade. Files are located in:
C:\Users\YourName\AppData\Local\Topten Software\Cantabile 3.0 (x64)\
Replace YourName with your account profile name.
The AppData folder may be hidden. The locale version of Windows uses a translated Users folder name.
Migration is usually seamless. Take a look at this thread.
Yes.
You shouldn’t. When v4 was released, I upgraded without needing to change any of my projects, including songs, racks, states, bindings…