EZ-903/904:
The EZ-903 Easy Educator is the 3-Dog version of the EZ-904 Easy Educator. Using the EZ-900 4-Dog transmitter makes it expandable to a 4-Dog system by simply adding a 4th collar receiver.
 
A modern remote trainer-designed with safety in mind, The EZ-904 has features like maximum stimulation setting and ‘lock and set’ to provide safety from accidental over stimulation of the dog. This medium range trainer is loaded with unique features making it the most humane and effective trainer available.
 
The bar shaped transmitter is especially well-suited for easy operation and is small providing stealth operation when the situation requires discreetness. Don’t be fooled by the small receiver size 2″”X1.5″”X1″” 2.4 ounce, it provides plenty of stimulation for most dogs even normal temperament 150 pound dogs respond to our blunt stimulation.
 
The unique COS (control of stimulation) technology provides very clean stimulation without the normal head jerking often found with other training collars. The boost feature is available to provide boosted stimulation from 1 to 60 and can be tailored individually for each dog.
 
The ergonomic and intuitive design allows the user watch the dogs rather than the remote. The night tracking light is controlled by the transmitter allowing the user to flash or continuously light up the collar receivers for locating your dogs after dark.
 
(Description copied from https://www.ecollar.com/product/ez-903-3-dog-easy-educator-remote-dog-trainer/)