From the EMSI Software Forums:

What’s new in a-squared Free 4.0?

  • We have added a second scan engine for the Scanner. The second engine comes from our partner, the famous Austrian antivirus company Ikarus, and is a pure antivirus engine. We expect much better detection rates with the dual engine mode. Compared with two standalone programs, the performance and memory usage of our bundled version is much better because we were able to combine them on a very low level and remove not necessary double signatures to save resources.
  • We have improved the a-squared scan engine: It loads 57% faster, uses 30% less RAM and scans 4% faster than v3.x.
  • The graphical user interface (GUI) has been refreshed completely. A new menu item for Quarantine and Update Logging has been added too.
  • Beyond that, we’ve added tons of minor tweaks and improvements to create a better usability.

To get the new 4.0 version, you can run an online update from an older version installation or download the latest setup file from Emsi Software.