Refactor LibBytes.readBytes4 for consistency

packages/contracts/src/contracts/current/protocol/AssetProxyOwner/AssetProxyOwner.sol

@@ -104,7 +104,7 @@ contract AssetProxyOwner is
         pure
         returns (bool)
     {
-        bytes4 first4Bytes = data.readFirst4();
+        bytes4 first4Bytes = data.readBytes4(0);
         require(REMOVE_AUTHORIZED_ADDRESS_SELECTOR == first4Bytes);
         return true;
     }

packages/contracts/src/contracts/current/test/TestLibBytes/TestLibBytes.sol

@@ -187,15 +187,19 @@ contract TestLibBytes {
         return b;
     }
 
-    /// @dev Reads the first 4 bytes from a byte array of arbitrary length.
-    /// @param b Byte array to read first 4 bytes from.
-    /// @return First 4 bytes of data.
-    function publicReadFirst4(bytes memory b)
+    /// @dev Reads an unpadded bytes4 value from a position in a byte array.
+    /// @param b Byte array containing a bytes4 value.
+    /// @param index Index in byte array of bytes4 value.
+    /// @return bytes4 value from byte array.
+    function publicReadBytes4(
+        bytes memory b,
+        uint256 index
+    )
         public
         pure
         returns (bytes4 result)
     {
-        result = b.readFirst4();
+        result = b.readBytes4(index);
         return result;
     }
 

packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol

@@ -430,16 +430,19 @@ library LibBytes {
         writeBytes32(b, index, bytes32(input));
     }
 
-    /// @dev Reads the first 4 bytes from a byte array of arbitrary length.
-    /// @param b Byte array to read first 4 bytes from.
-    /// @return First 4 bytes of data.
-    function readFirst4(bytes memory b)
+    /// @dev Reads an unpadded bytes4 value from a position in a byte array.
+    /// @param b Byte array containing a bytes4 value.
+    /// @param index Index in byte array of bytes4 value.
+    /// @return bytes4 value from byte array.
+    function readBytes4(
+        bytes memory b,
+        uint256 index)
         internal
         pure
         returns (bytes4 result)
     {
         require(
-            b.length >= 4,
+            b.length >= index + 4,
             GREATER_OR_EQUAL_TO_4_LENGTH_REQUIRED
         );
         assembly {

packages/contracts/test/libraries/lib_bytes.ts

@@ -459,17 +459,21 @@ describe('LibBytes', () => {
         });
     });
 
-    describe('readFirst4', () => {
+    describe('readBytes4', () => {
         // AssertionError: expected promise to be rejected with an error including 'revert' but it was fulfilled with '0x08c379a0'
         it('should revert if byte array has a length < 4', async () => {
             const byteArrayLessThan4Bytes = '0x010101';
             return expectRevertOrOtherErrorAsync(
-                libBytes.publicReadFirst4.callAsync(byteArrayLessThan4Bytes),
+                libBytes.publicReadBytes4.callAsync(
+                    byteArrayLessThan4Bytes,
+                    new BigNumber(0)),
                 constants.LIB_BYTES_GREATER_OR_EQUAL_TO_4_LENGTH_REQUIRED,
             );
         });
         it('should return the first 4 bytes of a byte array of arbitrary length', async () => {
-            const first4Bytes = await libBytes.publicReadFirst4.callAsync(byteArrayLongerThan32Bytes);
+            const first4Bytes = await libBytes.publicReadBytes4.callAsync(
+                byteArrayLongerThan32Bytes,
+                new BigNumber(0));
             const expectedFirst4Bytes = byteArrayLongerThan32Bytes.slice(0, 10);
             expect(first4Bytes).to.equal(expectedFirst4Bytes);
         });