What is the equivalent of keccak256(uint256(4) . uint256(1)) in web3?

Question

This line :

keccak256(uint256(4) . uint256(1)) in web3?

is taken from this Ethereum web page:

http://solidity.readthedocs.io/en/latest/miscellaneous.html#layout-of-state-variables-in-storage

How would I write this line using the web3 JS module?