The Lot-4 release is a combination of bug fixes along with a substantial refactoring of (the C# implementation of) the classes ESBehavior and ESMethod, and also of the AbstractSyntaxTreeNode classes. The refactoring was for the purpose of supporting the implementation of Traits.
Other than the bug fixes, the only change visible to those not directly dealing with the C# code is that instances of Behavior no longer support the specification of instance variables for their instances. That makes Behavior instances more like those in Squeak, Pharo or VisualWorks.
The new release can be downloaded from the Essence# site on CodePlex (which gets you the installer.) Alternatively, the code for the run time system (written in C#) can be obtained from theEssence# CodePlex site using Git, and the code for the Essence# Standard Library can be obtained from GitHub.