I showed this question to my three-year old son. His response - because he counted the apples one by one in each picture, passing "4" each time - was B, C and D. Hence, we need to take into account how children arrive at their conclusion, since they do not apply formal logic. The thought process is very different from the abstract approach a programmer might take when deciding whether his boolean expression is correctly formulated. I have worked with kids taking ability score tests, and sometimes even the very strictly standardised question sets with pictures, where the question itself was worded correctly, elicit unusual answers. But many children, if prompted, give a very compelling answer. "Odd one out" questions are especially problematic, and the question here is a variation of that.
It does not matter if the formal logic is the correct one, linguistically or mathematically, if children can not apply the rules in a conscious way. But of course we want a specific answer - at least for grading - and not find out how the child's analytical capabilities work exactly. To limit the risk of such unusual reasoning, best remove ambiguities, albeit without introducing logical loops that frame other questions in a more complicated way.
"Which picture shows exactly 4 apples" is the most accurate phrasing, and it avoids having to put "at least" into other questions to keep the logic consistent (see the "how many children do you have" example given in another reply).