ERC721 onERC721Received returns(bytes4)

`function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4);`

```python
contract ERC721TokenReceiver:
-> _data: bytes[256]
```
change to ?:
```python
 -> _data: bytes[4]
```
or
change to ?:
```python
 -> _data: bytes32
```