How do they know that an oral rinse product makes teeth 4x stronger?

Clinical, laboratory testing on actual test patients. Confirmed through microscopic examination.
Probably.

It is the same with the claim that energy drinks “give you energy”.
Because it is not properly defined, ergo no one knows what that means, it is legal to put on advertising and does not count as fraud, even if it is all bullshit.
So my take is, that it is a lie, clear and simple.

What’s needed is an automated machine that delivers equal blows to test subjects’ teeth.

Notice the choice of phrase. “Up to 4 x stronger.”

That leaves a lot of wiggle room.

When selling cars, you don’t say that doing x, will result in y. They say things like, anti-lock breaks are “designed” to not lock up… Or, the back seat, is “designed” to hold 3 people…

It’s rhetoric, that avoids litigation.

In a court room setting, one can answer any question, with “I don’t recall.”

Just examples of how to dance around 100% commitment, to a comment.

It doesn’t.