Is it OK to give your dog 0.4 ml of aspirin?

Yes, yes it is.

How much aspirin a dog can have depends on it's size and weight! If you are doubt about how much aspirin to give your dog, call your vet's office and ask them.

Regular strength buffered or baby aspirins should be used, not "safety coated" aspirin. Always give aspirin with food, to help avoid an upset stomach. If the dog starts to vomit, stop giving it aspirin.

Aspirin has blood thinning properties, so it can be dangerous in some dogs. For long term pain relief there are safer veterinary-specific alternatives.