chore(guide): fulfil error handler and trailers (#4087)

docs/Guides/Migration-Guide-V4.md

@@ -8,7 +8,39 @@ work after upgrading.
 
 ## Breaking Changes
 
-### Deprecation of `app.use()`
+### Error handling composition ([#3261](https://github.com/fastify/fastify/pull/3261))
+
+When an error is thrown in a async error handler function, 
+the upper-level error handler is executed if set.
+If there is not a upper-level error handler, the default will 
+be executed as it was previously.
+
+```
+import Fastify from 'fastify'
+
+const fastify = Fastify()
+
+fastify.register(async fastify => {
+  fastify.setErrorHandler(async err => {
+    console.log(err.message) // 'kaboom'
+    throw new Error('caught')
+  })
+  
+  fastify.get('/encapsulated', async () => {
+    throw new Error('kaboom')
+  })
+})
+
+fastify.setErrorHandler(async err => {
+  console.log(err.message) // 'caught' 
+  throw new Error('wrapped')
+})
+
+const res = await fastify.inject('/encapsulated')
+console.log(res.json().message) // 'wrapped'
+```
+
+### Deprecation of `app.use()` ([#3506](https://github.com/fastify/fastify/pull/3506))
 
 Starting this version of Fastify, we have deprecated the use of `app.use()`. We
 have decided not to support the use of middlewares. Both
@@ -70,3 +102,10 @@ properties: {
   }
 }
 ```
+
+### Add `reply.trailers` methods ([#3794](https://github.com/fastify/fastify/pull/3794))
+
+Fastify now supports the [HTTP Trailer] response headers.
+
+
+[HTTP Trailer]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Trailer