If the title is held as tenants in common (and no special designation on the deed is needed for this), then the deceased sibling's share can be willed to others; it does not automatically pass to the surviving owners. If, on the other hand, it was held as "joint tenants with rights of survivorship" (which must be specifically designated on the deed), then the 3 surviving siblings would become the sole owners of the property (i.e. the deceased sibling could not have willed their share).