Well, this certainly has created a very sticky situation. I am unsure why the four owners signed such a contract without the consent of the fifth owner, but the other owner has the legal right to file for partition. Normally partitions cannot be stopped, and they can be very lengthy, expensive, and nasty.
I would first examine the deed to determine if it is a joint tenants with rights of survivorship deed (JTWROS) or a joint tenancy in common (TIC) deed. Each deed will yield different results during a partition. With a JTWROS, all owners have an equal share regardless of contribution to purchase. So the proceeds would be distributed equally among all owners during a partiton. If the deed is a TIC deed, the ownership shares would be determined according to the amount each owner contributed toward the purchase price. If the property was inherited, then the proceeds most commony would be distributed equally.
Assuming that you have shares that are equal, or 1/5 shares, the proceeds from the sale of the property would be distributed equally. Now if the fifth person does not like the price that the other four of you have contracted, then you basically must either convience the fifth person to accept 1/5 of the contracted price or enter into the partition process. Filing for a partition is a legal right, and it cannot be stopped.