The 4t rule is a crucial guideline in sheet metal design. It stipulates that all features of the design, including slots, holes or notches, should be at least four times the material thickness away from any bend line. This prevents unwanted deformation and ensures the manufacturability of the designed part. The 't' in the rule represents the material thickness and the number '4' indicates the minimum distance in multiples of the material thickness. It's a rule of thumb that helps avoid design errors and enhances the efficiency of the bending operation.