Source and decoding
אלא אמר רבא: גזירה משום יום המעונן. אי הכי, אפילו בארבע שעות נמי לא ניכול! אמר רב פפא: ארבע זמן סעודה לכל היא
Rather Rava said: it is a decree because of a cloudy day. If so, even at four hours we should not eat? Rav Papa said: four is mealtime for everyone.
The Gemara discusses the hours for eating chametz on the eve of Pesach according to Rabbi Yehuda, after several amoraim offer explanations. Rava says there is a decree because on a cloudy day one can err about the hour. The Gemara asks why not forbid already at the fourth hour, and Rav Papa replies that the fourth is mealtime for everyone. The excerpt shows one answer and not the whole sugya.