Download the current Evimetry X-Ways AFF4 reader plugin, and copy the contents into the X-Ways installation folder. Make sure you have the Visual C++ 2015 Runtime installed.
The current FEX 4.0 build already integrates the Evimetry libAFF4 reader DLLs. This DLL contains a bug that has since been fixed in a later version of the DLL. We anticipate that this will make it into the next release of FEX. In the meantime replace the libaff4 DLL in Forensic Explorer with the one contained in the Evimetry libAFF4 reader DLL package.