After presenting the context of the teaching of mathematics in early school in Geneva we describe an activity inspired by some French researchers working within Brousseau's theory of didactical situations aiming at developing reasoning abilities about representing and reading a list taking into account the order of objects. We analyze this activity in terms of didactical variables to point out its specificities. We then give some results about its experimentation with students age 4 at the beginning of their first school year. It shows some great disparities in competences of students but also the possible progression to acquire some abilities, which are prerequisites for further conceptualization of the ordinal aspect of numbers.