Migrating Message Properties
In Mule 3, there are several property scopes that you can use for different purposes. In Mule 4, most of these scopes have been removed.
Invocation Properties
Invocation properties are now called variables in Mule 4. Their behavior is exactly the same as in Mule 3.
Inbound Properties, Outbound Properties, Inbound and Outbound Attachments
These scopes do not exist in Mule 4. See Intro Mule Message for more information about migration.