Upper left-hand quadrant: After R. Judah ben Bava was slain, they brought out R. Judah ben Dama for execution. The day was the eve of Shavuot. The Ten Martyrs
Upper right-hand quadrant: "The sound of the shofar grew stronger and stronger (lit: proceeds and becomes very strong)". This shofar is the left horn of Isaac's ram; the right horn is designated for the time of the messiah. S.Y. Agnon, Atem Re'item, p. 136.
Lower left-hand quadrant: Aaron held out his arm over the waters of Egypt, and the frogs came up and covered the land of Egypt. Exod. 8:2
Lower right-hand quadrant: R. Levi said: The Holy One appeared to them as though He were a statue with faces on every side, so that though a thousand men might be looking at the statue, they would be led to believe that it was looking at each one of them. Pesiqta de-Rav Kahana