Why does Winston write that “Freedom is the freedom to say that two plus two equals four? Why is this basic fact so important to him?

The party can effectively create truth. They can change the past and control the present. They have the ability to create reality. Winston desperately tries to hold on to fundamental truths like the law of gravity and 2+2=4. Winston wants to hold on to these truths when faced with whatever reality the party chooses to impose on him.