Set-off and carry forward of race-horse losses limited to same activity, four assessment years, timely return required. Section 74A allows set-off of losses from owning and maintaining race-horses only against income from that same activity, with unabsorbed losses carried forward for four assessment years. Carry forward is permitted only if the return is filed before the due date under section 139(1). Loss equals revenue expenditure wholly and exclusively for maintaining race-horses; where stake income exists, loss is expenditure less stake income. Section 74A applies only to race-horses; other animals fall under section 72.