If you don't like vB5, then you may choose to install vB4 instead. However, you cannot use the database from vB5 for a vB4 site - you would need to start over with a fresh install.
If I buy vb5 and decide I hate it and want to go back to 4.x. Is that possible with the license for vb5?
what i would do and you should do this with any new version of vb is install vb5 on a test forum first and see how you like it and how the functionality is ...