 For example, "doghouse" is a dative compound, a house "for" a dog.It would be called a refers to the fourth case, that is, the dative).Incidentally, the word "tatpuruṣa" is itself a tatpuruṣa (meaning a "that-man", in the sense of "that person's man", meaning someone's agent), while "caturthītatpuruṣa" is a "karmadhāraya", being both dative and a tatpuruṣa.