Siman 406

One who went outside the techum, who has only four amot, and needed to relieve himself—he may go out from there until he finds a modest place to relieve himself. This is even if he went out intentionally, since he is now under duress in this matter. And if he went out unknowingly, as was explained in the previous siman that if they returned him to within the techum it is as if he did not go out, the sages gave him advice when he needs to relieve himself—that he should approach the side of his techum, and if he does not find a modest place until his techum—he shall enter his techum, and after he has entered it is as if he did not go out, since he entered with permission, and it is like they returned him. This is when he did not find a modest place before his techum. But if he found a modest place beforehand—he shall not enter his techum, but rather he shall relieve himself there, and distance himself from the place where he relieved himself until the odor dissipates, and there he has four amot. And if he distanced himself because of the odor toward the side of his techum and entered his techum—it is as if he did not go out. This is when he went out under duress, but intentionally—it does not avail him that he enters his techum, for he still has only four amot, as I wrote in the previous siman.

That which we have said regarding one who needs to relieve himself, some say that this same law applies even if he needs to urinate. And some say specifically for bowel movements, but to urinate—he may do so in any place and does not require a modest place for this, for they are absorbed in their place, and one may urinate in the presence of many. (Bekhorot 44b) And perhaps when he needs to pray or to recite Birkat HaMazon, where it is not possible within four amot of the urine—he can find a modest place.