Microsoft Patterns & Practices released the Enterprise Library 4.0 CTP. You can download it here.
If you use Enterprise Library quite a bit, at a minimum I recommend downloading the change log to see if they missed a bug or issue that you were hoping to have addressed in the latest release.
It is nice to at least see the following two fixes as mentioned in the change log:
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In version 4.0, Enterprise Library has the Allow Partially-Trusted Caller attribute (APTCA) on all assemblies. This means that you can call the methods of Enterprise Library and the application blocks from an application running in a partial trust environment. You can do this with the signed assemblies provided with Enterprise Library. There is no longer any requirement, as there was in version 3.x, to recompile the source code and the source code for Object Builder then either use the unsigned binaries or strong-name them yourself.
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The [Logging] application block now allows you to specify that the RollingFlatFileTraceListener will start a new file at midnight every day. To use this behavior, set the value of the RollInterval property of the RollingFlatFileTraceListener to Midnight.
When Enterprise Library 4.0 is officially released, it will also have optional support for using Unity IoC to wire up the various application blocks.
If you haven't used Enterprise Library, I have numerous Enterprise Library 2.0 Tutorials and Enterprise Library 3.0 Tutorials that I have put together over the past couple of years that may be worth reading.