The phrase "at a stretch" and "at the most" do not have the same meaning. The woman in the movie was likely using the phrase "at a stretch" to convey the idea that they could stretch or extend their available meals to last for a longer period, beyond the usual or expected amount of time. In this context, it suggests making the meals last longer than originally intended. On the other hand, "at the most" implies a maximum limit or the highest possible quantity. So, the Spanish subtitles may not accurately capture the intended meaning of "at a stretch." "4 at the most" suggests that there won't be more than four meals, whereas "4 at a stretch" implies the possibility of making those four meals last longer than expected. In this case, a more accurate translation for "at a stretch" in this context might be something like "4, extendiéndolos lo más posible" in Spanish, which conveys the idea of stretching or extending the meals to their maximum duration.

To "stretch" can mean to make a special effort, or to work hard. "At a stretch" is used when you've set a limit, but it isn't an absolute limit. You are naming a second number which is possible, with difficulty. For example, "When you are learning a new language, how many new words a day can you learn? The answer is about ten words a day--twenty at a stretch."