Siman 157
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א כְּשֶׁיַּגִּיעַ שָׁעָה רְבִיעִית יִקְבַּע סְעוּדָתוֹ; וְאִם הוּא תַּלְמִיד חָכָם וְעוֹסֵק בְּלִמּוּדוֹ, יַמְתִּין עַד שָׁעָה ו', וְלֹא יְאַחֵר יוֹתֵר דְּהָוֵי כְּזוֹרֵק אֶבֶן לַחֵמֶת אִם לֹא טָעַם מִדִּי בְּצַפְרָא.
1 Laws of When To Establish One's Meal. Containing 1 Se'if: When the fourth hour [of the day] arrives, one should establish his meal. And if he is a Torah scholar and he is occupied with his learning, he should wait until the sixth hour, but he should not delay more than that for it is considered like throwing a stone into a [wine] pouch if he did not eat anything that morning.