Support for old versions is generally discontinued with the release of the latest version as it isn't support for a version specifically but for the product, avast 4 Home/Pro.
So with 4.8 you would be notified that there is a program version update available and it would be your choice to download an install it.
The exception to this is for old OSes, win95/98/ME which in this case will have to stay on version 4.8 as the new version 5 doesn't support these OSes. So 4.8 support will continue for these OSes at least to the end of this year (after that nothing has been made known).
So what is your OS ?
If you are able to update to version 5 but choose not to then the new functionality that will be available will not be included in version 4.8, so those that stay with 4.8 won't see any benefit of the new developments.