Equivalence of families of singular schemes on threefolds and on ruled fourfolds

The main purpose of this paper is twofold. We first want to analyze in details the meaningful geometric aspect of the method introduced in the previous paper [12], concerning regularity of families of irreducible, nodal “curves” on a smooth, projective threefold $X$. This analysis highlights several fascinating connections with families of other singular geometric “objects” related to $X$ and to other varieties. Then, we generalize this method to study similar problems for families of singular divisors on ruled fourfolds suitably related to $X$.