Right, let's see how this law violates the Fourth Amendment.
Case 1. A driver is not wearing a seat belt. A policeman looks in through the windscreen and notices that the driver is not wearing a seat belt. He flags down the car and charges the driver with an offense.
No violation. The 4th amendment doesn't protect people from being looked at by a police officer while they happen to be breaking the law.
Case 2. A driver is not wearing a seat belt. A policeman looks towards the car but is unable to tell whether the driver is wearing a seat belt or not. Having no reason to suspect a crime is being committed, he takes no action.
No violation. Obviously.
Case 3. A driver is not wearing a seat belt. A policeman is unable to see whether he is wearing one or not, but decides to stop the car and look inside. He charges the driver, but the case is dismissed because the policeman is judged to have violated the 4th Amendment.
A violation has occurred, but is handled correctly by the law.
I would conclude that the law itself doesn't violate the 4th Amendment in any way whatsoever, because the law itself does not mandate or permit any particular means of enforcement. Whether you agree with it, or whether it's a good law, is a different question; but the claim, specifically, that it violates the 4th Amendment is groundless.
Dave