To determine which group is not included under the term 'Hindu' in the Hindu Marriage Act:
1. The Hindu Marriage Act, 1955, applies to Hindus, Sikhs, Buddhists, and Jains, treating them under the broader Hindu law framework.
2. Parsis, however, are governed by the Parsi Marriage and Divorce Act, 1936, as a distinct religious community.
3. Thus, Parsis are excluded from the definition of 'Hindu' in this context.
Therefore, the correct answer is (4) Parsis.