For one, it's so much quieter than the 3. The drawer was redesigned to be much easier to replace the bag. Sleep timers can now be defined with start and end times for both weekdays and weekends. The 3 only allows an 8 hour sleep with a given start time. The cat detection sensors are much better at detecting cats. There is now a litter level sensor. The cat weight sensor is probably nice, but with 4 cats, they're too close in weight to get any good use out of it. It also looks nicer.