Former Federal Reserve Board Chairman Paul Volcker at the Senate on May 9, 2012. - 

It's been four years since Congress passed the Volcker Rule, a sweeping law aimed at fixing the financial system following its great collapse. But regulators still haven’t figured out how to implement it.

Put simply, the rule send this message to big financial companies: It's ok to place financial bets with client money, but not with bank funds. The rationale? If bank bets go bad, it could bring down the whole system and leave taxpayers picking up the pieces.

Allan Sloan, senior editor-at-large at Fortune Magazine, joins Marketplace Morning Report host David Brancaccio to discuss a new proposal.