Prepare 2.0b4.

CHANGELOG.md

 Change Log
 ==========
 
-Version 2.0.0-beta4 *(In Development)*
---------------------------------------
+Version 2.0.0-beta4 *(2016-02-04)*
+----------------------------------
 
  * New: `Call` instance is now passed to both `onResponse` and `onFailure` methods of `Callback`. This aids
    in detecting when `onFailure` is called as a result of `Call.cancel()` by checking `Call.isCanceled()`.

README.md

@@ -14,12 +14,12 @@ Download [the latest JAR][2] or grab via Maven:
 <dependency>
   <groupId>com.squareup.retrofit2</groupId>
   <artifactId>retrofit</artifactId>
-  <version>2.0.0-beta3</version>
+  <version>2.0.0-beta4</version>
 </dependency>
 ```
 or Gradle:
 ```groovy
-compile 'com.squareup.retrofit2:retrofit:2.0.0-beta3'
+compile 'com.squareup.retrofit2:retrofit:2.0.0-beta4'
 ```
 
 Snapshots of the development version are available in [Sonatype's `snapshots` repository][snap].