You should know why you believe 2+2=4. If the reason why you believe it ceases to be valid, you should be ready to change your belief. In other words, I can imagine observations which would convince me that 2+2=3 (for more details refer to http://lesswrong.com/lw/jr/how_to_convince_me_that_2_2_3/)
You can never be 100% sure of something because that would take away from you the possibility of ever updating your beliefs if new evidence comes. And rationality is about creating beliefs based on evidence.