It's not like I have a suitcase of money in my trunk and I've been pulled over by the police without cause and I can refuse to give consent.
Therefore I don't think the 4 amendment applies.
Isn't the argument for CAF that the mere possession of the large sum of money is just cause to confiscate it? That's why a time limit would work. DUring that time period, they could try to get a warrant. The judge would not give a warrant if that is the only evidence. Then the assets would be returned.