Why did later logicians add a fourth figure of syllogisms?

Later logicians identified a fourth figure of syllogisms because there is a distinct logical pattern—where the middle term is predicated of the major term and the minor term is predicated of the middle term—that Aristotle recognized but did not group under a separate figure in his original system.1

While some later logicians classified these patterns as a separate fourth figure, others, such as Aristotle's successor Theophrastus, chose to incorporate them into the first figure by reinterpreting its definition. Although tradition has often credited the physician Galen with discovering the fourth figure, he actually rejected the classification. Ultimately, the addition of a fourth figure serves to categorize all 256 possible syllogistic patterns, of which 24 are considered valid.21345