Advantages of using a 2 x 4 table to test for gene-environment interactions are described in this slide. Perhaps the most important is the last one listed. You will be able to readily determine if the number of individuals in any of the cells in the 2 x 4 table is small. Small sample sizes are a major problem when testing for gene-environment interactions because cases and controls are each stratified by 2 factors. This can result in unstable (and overestimated) ORs.