Updated the Wolverine 4.0 migration guide. Closes GH-1453

docs/guide/migration.md

@@ -1,5 +1,18 @@
 # Migration Guide
 
+## Key Changes in 4.0
+
+4.0 had very few breaking changes.
+
+* Wolverine dropped all support for .NET 6/7
+* The previous dependencies on Oakton, JasperFx.Core, and JasperFx.CodeGeneration were all combined into a single [JasperFx](https://github.com/jasperfx/jasperfx)
+  library. There are shims for any method with "Oakton" in its name, but these are marked as `[Obsolete]`. You can pretty well do a find and replace for "Oakton" to "JasperFx"
+* Internally, the full "Critter Stack" is trying to use `Uri` values to identify databases when targeting multiple databases in either a modular monolith approach or with multi-tenancy
+* Many of the internal dependencies like Marten or AWS SQS SDK Nugets were updated
+* The signature of the Kafka `IKafkaEnvelopeMapper` changed somewhat to be more efficient in message serialization
+* Wolverine now supports [multi-tenancy through separate databases for EF Core](/guide/durability/efcore/multi-tenancy)
+* The Open Telemetry span names for executing a message are now the [Wolverine message type name](/guide/messages.html#message-type-name-or-alias)
+
 ## Key Changes in 3.0
 
 The 3.0 release did not have any breaking changes to the public API, but does come with some significant internal