With diad 3 the keys are a lot larger and easier to see and read until you get so used to the keyboard that you don't have to look. The toggle key is simply a shift key that allows the same key to have two functions, much like a shifting key on the typewriter or keyboard allows small and large case letters. Diad 4 is more sophisticated with more features, but i liked the big keys on the diad 3. Other than those few differences, there should be little difficulty for you in the transition.