The four propositions he refers to are a standard Buddhist analysis of the possibilities of the nature of reality. The first proposition is existence. The second is nonexistence. The third is both, and the fourth is neither. The hundred negations are the various ways that you can take those up and turn, negate, obviate, transform each of those possibilities.
Nonexistence is slandered by underestimation.
Both existence and nonexistence is slandered by contradiction. Neither existence nor nonexistence is slandered by intellectual fabrication. If you say you understand clearly and thoroughly, nothing is not it. We can use the psychoanalytic understanding of repression to help us understand anatta, and viceversa. We can see how subtle issues of confidence and self-trust are within a tradition that recognizes anatta.