If you installed Version 5 in a new folder, you should be able to still run Version 4. Just don't run them at the same time since they will be competing for connections if they are configured to use the same servers.
The installer for 5.01 does not allow this but, the original 5.0 installer would allow you to install 5.0 in the same folder as Version 4. If you did this, reverting back to Version 4 can be very complicated. Unless you had a complex configuration for V4, it may be easier to just uninstall V5 and install V4 and start over.
If you want to try to go back:
1) Uninstall V5. MAKE SURE that you answer "No" to "Remove Default
NewsBin Data Folder?". Your filters.xml file will still get deleted,
if you need to retain filters, copy this file out of the folder before
2) If you requested to upgrade from 4 to 5.0 then version 4 was uninstalled.
2a) If you chose "No" to "Remove Version 4 Data Files" then you can probably go back, you'll have to re-install version 4 over the same folder but your original settings will be retained.
2b) If you chose "Yes" to "Remove Version 4 Data File then you might
as well uninstall V5 then reinstall V4 and start over
3) Copy your filters.xml file back into the install folder if you saved it.
4) When you start V4, cancel out of the setup screens and go under
File/Open and open your original V4 configuration file (ends with a
5) You will have to re-enter your registration key.