EY lays out possible evidence for 2+2=3 here. Imagine you believe at 50% level that someone will cause you to view that evidence tomorrow. Hypnosis, or some other method. Applying Bayes rule like EY seems to be applying it here, you should evaluate right now at most a 50% chance that 2+2=4. I think the rational thing to do in that situation (where putting the earplugs together does in fact show 2+2 equaling 4), is to believe that 2+2=4, with around the same much confidence as you do now.

Why do you think that is the correct thing to do in that situation?

Here, in this real situation, yes you should trust your current counting abilities. But if you believe with 50% confidence that, within 24 hours, someone will be able to convince you that your ability to count is fundamentally compromised, you also don't place a high level of confidence on your ability to count things correctly - no more than 50%, in fact.

"I can count correctly" and "[someone can demonstrate to me that] I'm counting incorrectly" are mutually exclusive hypotheses. Your confidence in the two ought not to add up to more than 1.