This question involves the interplay between Section 25 and Section 27 of the Indian Evidence Act.
Section 25: Makes any confession made to a police officer inadmissible. Statements I, II, and III are confessions of guilt (admitting to the robbery, participation, and receiving proceeds). As such, they are inadmissible.
Section 27: Provides an exception. It allows "so much of such information... as relates distinctly to the fact thereby discovered" to be proved. The fact discovered here is the 'share' (the property). The information leading to this discovery is the statement about where the property is and the willingness to produce it. Therefore, Statement IV, which leads to the discovery, is admissible.
The question asks what is not admissible. Statements I, II, and III, which are pure confessions of the crime, are not admissible.