diff --git a/src/Data/Word/ToolsYj.hs b/src/Data/Word/ToolsYj.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Word/ToolsYj.hs
@@ -0,0 +1,59 @@
+{-# OPTIONS_GHC -Wall -fno-warn-tabs #-}
+
+module Data.Word.ToolsYj (
+
+	Word4
+
+	) where
+
+import Control.Arrow
+import Data.Bits
+import Data.Word
+
+newtype Word4 = Word4_ { unWord4 :: Word8 }
+
+instance Bits Word4 where
+	Word4_ w1 .&. Word4_ w2 = Word4_ $ w1 .&. w2
+	Word4_ w1 .|. Word4_ w2 = Word4_ $ w1 .|. w2
+	Word4_ w1 `xor` Word4_ w2 = Word4_ $ w1 `xor` w2
+	complement = Word4_ . (.&. 0xf) . complement . unWord4
+	shift (Word4_ w) i = Word4_ . (.&. 0xf) $ shift w i
+	rotate (Word4_ w) i = Word4_ $ shiftR w' 4 .|. (w' .&. 0xf)
+		where
+		w' = (`rotate` i) $ (.&. 0xf) w
+	bitSize _ = 4; bitSizeMaybe _ = Just 4; isSigned _ = False
+	Word4_ w `testBit` i = w `testBit` i
+	bit i = Word4_ . (.&. 0xf) $ bit i
+	popCount (Word4_ w) = popCount $ (.&. 0xf) w
+
+instance FiniteBits Word4 where
+	finiteBitSize _ = 4
+
+instance Bounded Word4 where minBound = 0; maxBound = 0xf
+
+instance Enum Word4 where
+	toEnum i
+		| 0 <= i && i <= 0xf = fromIntegral i
+		| otherwise = error $
+			"Enum.toEnum{Word4}: tag " ++
+			show i ++ " is outside of bounds (0,15)"
+	fromEnum = fromIntegral
+
+instance Num Word4 where
+	Word4_ w1 + Word4_ w2 = Word4_ . (.&. 0xf) $ w1 + w2
+	Word4_ w1 * Word4_ w2 = Word4_ . (.&. 0xf) $ w1 * w2
+	abs = id
+	signum _ = 1
+	negate = Word4_ . (.&. 0xf) . negate . unWord4
+	fromInteger = Word4_ . (.&. 0xf) . fromInteger
+
+instance Real Word4 where
+	toRational = toRational . (.&. 0xf) . unWord4
+
+instance Integral Word4 where
+	toInteger = toInteger . (.&. 0xf) . unWord4
+	Word4_ w1 `quotRem` Word4_ w2 =
+		(Word4_ . (.&. 0xf) *** Word4_ . (.&. 0xf)) $ w1 `quotRem` w2
+
+instance Eq Word4 where Word4_ w1 == Word4_ w2 = (w1 .&. 0xf) == (w2 .&. 0xf)
+instance Ord Word4 where Word4_ w1 <= Word4_ w2 = (.&. 0xf) w1 <= (.&. 0xf) w2
diff --git a/tools-yj.cabal b/tools-yj.cabal
--- a/tools-yj.cabal
+++ b/tools-yj.cabal
@@ -5,7 +5,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           tools-yj
-version:        0.1.0.42
+version:        0.1.0.43
 synopsis:       Tribial tools
 description:    Please see the README on GitHub at <https://github.com/YoshikuniJujo/tools-yj#readme>
 category:       Tools
@@ -46,6 +46,7 @@
       Data.Tuple.ToolsYj
       Data.Word.Adler32
       Data.Word.Crc32
+      Data.Word.ToolsYj
       Foreign.C.ByteArray
       Text.Show.ToolsYj
   other-modules: