diff --git a/README.md b/README.md
index ddd3c0c..a58bbe7 100644
--- a/README.md
+++ b/README.md
@@ -289,3 +289,24 @@ Gen proto and client file
 ```erlang
 rebar3 etcd gen
 ```
+
+Migration from eetcd 0.3.6 to 0.4.x
+-----
+
+eetcd 0.4.x now dependents on Gun 2.0, which introduced some broken changes,
+and propagate to eetcd.
+
+The transport options are split into `tcp_opts` and `tls_opts`. As a result the
+functions `eetcd:open/4,5` have been replaced with `eetcd:open/5,6`.
+
+Likewise, the transport options for `eetcd_maintenance` APIs are split into
+`tcp_opts` and `tls_opts` as well.
+
+- The function `eetcd_maintenance:defragment/3` has been replaced with `eetcd_maintenance:defragment/4`.
+- The function `eetcd_maintenance:status/3` has been replaced with `eetcd_maintenance:status/4`.
+- The function `eetcd_maintenance:has_kv/4` has been replaced with `eetcd_maintenance:has_kv/5`.
+
+New options `{domain_lookup_timeout, Interval}` and `{tls_handshake_timeout, Interval}`
+have been added for `eetcd:open/5,6`. Alone with the prior `{connect_timeout, Interval}`,
+it allows the underlining Gun library to get separate events when connecting,
+the domain lookup, connection and TLS handshakes.
diff --git a/src/eetcd.app.src b/src/eetcd.app.src
index c2a69cf..ce828eb 100644
--- a/src/eetcd.app.src
+++ b/src/eetcd.app.src
@@ -1,7 +1,7 @@
 {application, eetcd,
     [
         {description, "ETCD V3 client"},
-        {vsn, "0.3.6"},
+        {vsn, "0.4.0"},
         {registered, [eetcd_sup, eetcd_conn_sup, eetcd_lease_sup]},
         {mod, {eetcd_app, []}},
         {applications, [kernel, stdlib, gun]},