Can you actually demonstrate that 2+2=4? That proof is notoriously difficult, so even if you are a good mathematician you may make a mistake.

Are you positive that your brain works fine? Are you sure you are not hallucinating? Have you considered that you may be living in a simulation run by an evil demon?

That said, of course our confidence that 2+2=4 is very, very, very high. But we cannot logically exclude the (very remote) possibility that we are wrong. And what do we lose, exactly, by acknowledging such possibility?