When identifying the odd one out, pay attention to categories such as botanical classification versus culinary usage, as these can often differentiate similar items.
Let's analyze the given options:
- Apple, Orange, and Pineapple are all fruits that grow on trees.
- Tomato, while botanically a fruit, is often categorized as a vegetable due to its culinary use.
Thus, Tomato is the odd one out because, unlike the others, it is commonly treated as a vegetable.
Therefore, the correct answer is \( \boxed{4} \).