4. The contract has been signed (by me).
5. The tickets have been bought by these men.
6. This teddy bear has been bought in that shop (by me).
7. Vicky's hat has been found (by her).
8. My bag has been lost (by me).
9. The report has been finished (by us). 

4. I have signed the contract.

=> The contract has be signed by me.

5. These men have bought the tickets.

=> The tickets have be bought by these men.

6. I have bought this teddy bear in that shop.

=> This teddy bear in that shop has be bought by me. 

7. Vicky has found her hat.

=> Vicky's hat has be found by her.

8. I have lost my bag.

=> My bag has be lost (by me).

9. We have finished the report.

=> The report has be finished (by us).