diff --git a/lib/fluent/event.rb b/lib/fluent/event.rb
index 97711f4621..d1216fe148 100644
--- a/lib/fluent/event.rb
+++ b/lib/fluent/event.rb
@@ -294,7 +294,7 @@ def to_msgpack_stream(time_int: false, packer: nil)
       super
     end
 
-    def to_compressed_msgpack_stream(time_int: false)
+    def to_compressed_msgpack_stream(time_int: false, packer: nil)
       # time_int is always ignored because @data is always packed binary in this class
       @compressed_data
     end
diff --git a/test/plugin/test_output_as_buffered_compress.rb b/test/plugin/test_output_as_buffered_compress.rb
index b698aeafee..1a78332d47 100644
--- a/test/plugin/test_output_as_buffered_compress.rb
+++ b/test/plugin/test_output_as_buffered_compress.rb
@@ -35,6 +35,16 @@ def format(tag, time, record)
       @format ? @format.call(tag, time, record) : [tag, time, record].to_json
     end
   end
+
+  def self.dummy_event_stream
+    Fluent::ArrayEventStream.new(
+      [
+        [event_time('2016-04-13 18:33:00'), { 'name' => 'moris', 'age' => 36, 'message' => 'data1' }],
+        [event_time('2016-04-13 18:33:13'), { 'name' => 'moris', 'age' => 36, 'message' => 'data2' }],
+        [event_time('2016-04-13 18:33:32'), { 'name' => 'moris', 'age' => 36, 'message' => 'data3' }],
+      ]
+    )
+  end
 end
 
 class BufferedOutputCompressTest < Test::Unit::TestCase
@@ -60,16 +70,6 @@ def waiting(seconds)
     end
   end
 
-  def dummy_event_stream
-    Fluent::ArrayEventStream.new(
-      [
-        [event_time('2016-04-13 18:33:00'), { 'name' => 'moris', 'age' => 36, 'message' => 'data1' }],
-        [event_time('2016-04-13 18:33:13'), { 'name' => 'moris', 'age' => 36, 'message' => 'data2' }],
-        [event_time('2016-04-13 18:33:32'), { 'name' => 'moris', 'age' => 36, 'message' => 'data3' }],
-      ]
-    )
-  end
-
   TMP_DIR = File.expand_path('../../tmp/test_output_as_buffered_compress', __FILE__)
 
   setup do
@@ -89,20 +89,34 @@ def dummy_event_stream
   end
 
   data(
-    handle_simple_stream: config_element('buffer', '', { 'flush_interval' => 1, 'compress' => 'gzip' }),
-    handle_stream_with_standard_format:  config_element('buffer', 'tag', { 'flush_interval' => 1, 'compress' => 'gzip' }),
-    handle_simple_stream_and_file_chunk: config_element('buffer', '', { '@type' => 'file', 'path' => File.join(TMP_DIR,'test.*.log'), 'flush_interval' => 1, 'compress' => 'gzip' }),
-    handle_stream_with_standard_format_and_file_chunk:  config_element('buffer', 'tag', { '@type' => 'file', 'path' => File.join(TMP_DIR,'test.*.log'), 'flush_interval' => 1, 'compress' => 'gzip' }),
+    :buffer_config,
+    [
+      config_element('buffer', '', { 'flush_interval' => 1, 'compress' => 'gzip' }),
+      config_element('buffer', 'tag', { 'flush_interval' => 1, 'compress' => 'gzip' }),
+      config_element('buffer', '', { '@type' => 'file', 'path' => File.join(TMP_DIR,'test.*.log'), 'flush_interval' => 1, 'compress' => 'gzip' }),
+      config_element('buffer', 'tag', { '@type' => 'file', 'path' => File.join(TMP_DIR,'test.*.log'), 'flush_interval' => 1, 'compress' => 'gzip' }),
+    ],
   )
-  test 'call a standard format when output plugin adds data to chunk' do |buffer_config|
+  data(
+    :input_es,
+    [
+      FluentPluginOutputAsBufferedCompressTest.dummy_event_stream,
+      # If already compressed data is incoming, it must be written as is (i.e. without decompressed).
+      # https://github.com/fluent/fluentd/issues/4146
+      Fluent::CompressedMessagePackEventStream.new(FluentPluginOutputAsBufferedCompressTest.dummy_event_stream.to_compressed_msgpack_stream),
+    ],
+  )
+  test 'call a standard format when output plugin adds data to chunk' do |data|
+    buffer_config = data[:buffer_config]
+    es = data[:input_es].dup # Note: the data matrix is shared in all patterns, so we need `dup` here.
+
     @i = create_output(:async)
     @i.configure(config_element('ROOT','', {}, [buffer_config]))
     @i.start
     @i.after_start
 
     io = StringIO.new
-    es = dummy_event_stream
-    expected = es.map { |e| e }
+    expected = es.dup.map { |t, r| [t, r] }
     compressed_data = ''
 
     assert_equal :gzip, @i.buffer.compress
@@ -138,7 +152,7 @@ def dummy_event_stream
     @i.after_start
 
     io = StringIO.new
-    es = dummy_event_stream
+    es = FluentPluginOutputAsBufferedCompressTest.dummy_event_stream
     expected = es.map { |e| "#{e[1]}\n" }.join # e[1] is record
     compressed_data = ''