A conference design is a rectangular matrix with orthogonal columns, one zero in each column, at most one zero in each row, and −1s and +1s elsewhere. A definitive screening design can be constructed by folding over a conference design and adding a row vector of zeroes. We prove that, for a given even number of rows, there is just one isomorphism class for conference designs with two or three columns. Next, we derive all isomorphism classes for conference designs with four columns. Based on our results, we propose a classification criterion for definitive screening designs based on projections into four factors and illustrate its potential by studying designs with 24 factors. * Supported by the Research Foundation -Flanders FWO MSC 2010 subject classifications: Primary 62K20; secondary 05B20, 94C30