Yes, deduction u/s 24 & 80C will be allowed to all four family members in the ratio of there ownership as per registry papers. But there is one condition that all the other three members of the family must repay the amount in the ratio of there ownership to the member who is making repayment of the loan from his account. For Example let the four members be A,B,C & D. For now, let the share of ownership be equal i.e. 25% each. Mr. A has paid 100/- from his a/c as repayment of loan. Now for taking deduction u/s 24 & 80C all the other members, i.e. Mr. B,C & D have to repay 25/- each (25% of 100/-) to Mr. A.