The sandbox currently incorporated in CIS is not yet bulletproof. Currently it will allow a program to drop files outside of the sandbox. Please see the guide in the bottom of my reply for more information about how the sandbox works.
I believe that this is what they are aiming for (I hope) however it's not quite there yet. Perhaps this is the reason the update to V4 hasn't been pushed to the V3 users as of yet. Personally the Sandbox feature in V4 feels very much like a Beta to me.Until the sandbox issue is solved I'd stick to using Sandboxie or Returnil.