If your HP Office Jet 4315 All-in-One Fax Machine is not receiving the fax properly,then you will have to uninstall its driver and then properly install it particularly the software for fax.
How Do You Receive A Fax From A HP Office Jet 4315 All-in-One Fax Machine?
Also make sure it's connected to a phone line and the phone is allowed to ring uninterrupted while transmitting.