Bumped version to 5.0.0-alpha4

docs/reference/index.asciidoc

@@ -1,7 +1,7 @@
 [[elasticsearch-reference]]
 = Elasticsearch Reference
 
-:version:       5.0.0-alpha3
+:version:       5.0.0-alpha4
 :major-version: 5.x
 :branch:        master
 :jdk:           1.8.0_73