The term 'Marmoset Relative' can be summarized by the crossword solution of 'Titi', which is a four-letter word. Titi refers to a type of monkey that is closely related to the marmoset, a small species of monkey native to South America. Titis are also found in South America and are known for their long, bushy tails and their social behavior. They live in groups and are highly vocal, making them popular pets.