When updating to use NUnit 4.0.1 the following exception occurs:
System.TypeLoadException : Method 'get_Description' in type 'Org.XmlUnit.Constraints.CompareConstraint' from assembly 'xmlunit-nunit3-constraints, Version=2.9.2.240, Culture=neutral, PublicKeyToken=e7d7e3088fd452f6' does not have an implementation.

NUnit4 requires constraints to have a Description getter.