Quick take
Choose Hydro 3 when cordless simplicity matters most. Choose Hydro 4 SE when continuous power and more aggressive floor/wall/waterline cleaning are worth managing a cable.
Cordless vs corded
Hydro 3 is positioned as a cordless option, while Hydro 4 SE uses a power cable.
Maintenance and charging
Cordless ownership means battery charging and eventual battery aging. Corded ownership avoids battery runtime limits but adds cable handling.
Pool size and cleaning style
Larger or more demanding pools may benefit from continuous corded operation. Small pools and quick cycles can favor cordless convenience.
Price
Hydro 3 is usually the lower-cost entry point, while Hydro 4 SE moves up for broader cleaning features.