Options 1 (Gibbon), 2 (Gorilla), and 4 (Orangutan) are all species of apes. Apes are a group of primates that includes gibbons, gorillas, and orangutans, among others. They are known for their intelligence and lack of a tail.

Option 3 (Langur), however, is not an ape but a type of Old World monkey. While both apes and monkeys are primates, they are different in terms of physical characteristics and evolutionary lineage. Apes tend to be larger and have a more developed brain compared to monkeys. Also, unlike apes, most monkeys have tails. Thus, langur is the only one among the options that is not an ape.