Version 5 brings several improvements, such as a smaller footprint that uses fewer system processes, quicker scan speeds, a "Behavioral Shield" for threats not present in Avast's definitions, and a revamped GUI. Meanwhile, the Pro and Internet Security options add a script shield, a sandbox, and a command line scanner. As a suite, of course, Internet Security also has an anti-spam filter and built-in firewall.
The official Avast blog notes that if you're using version 4.8, updating to 5 is painless. You can download the base software here free of charge, but the Pro and Internet Security solutions are $39.95 and $59.95, respectively.