What happens if you brush with 4 year old toothpaste?

Nothing :): Toothpaste is just detergent, flavoring, filler material, and fluoride. Fluoride ions do not decompose, and old detergent is still detergent. Brush religiously twice a day until you need new toothpaste!

Nothing terrible.: The only problem could be the Fluoride in the toothpaste may have lost its effectiveness and so it won't help protect you against cavities forming, but there's nothing poisonous or dangerous with old toothpastes.

It works: Expiration dates are mainly about the efficacy of Fluoride in the toothpaste and not the physical characteristics that make it a good surfactant and mild abrasive. Old toothpaste is not dangerous. One tube of toothpaste lasting over a year.

Nothing: The action and frequency use of tooth brush is important not the tooth paste. Thinking of tooth paste as lubricant. But they add some other thing to help you have a healthier teeth.