1. Yes, all the four people are the absolute owners of the property and can sell the property to any prospective buyer.
2. If all the four legal heirs are participating in the sale of the property, there will be no need for the execution of Relinquishment Deed.
3. Only at the time of registering the property, all the four have to be physically present. In case it's not possible for a legal heir to be physically present during registration of the property, he/she may authorise one of the family members to represent him/her by giving POA, to act on his/her behalf.
4. The documents which are required will be Mother Deed (how your grandfather obtained property), WILL of your grandfather, his death certificate, your father's death certificate, family tree, Aadhar Cards of all the family members.
5. You have to obtain probate of the WILL from the jurisdictional competent court.