The difference is in control. The Rialto 600 can be controlled via IR and RS232 control. The Rialto 400 can not.
The Rialto 600 also has pre-amp volume control and 3 discrete inputs with a crossover.
The Rialto 400 has signal sense, and a priority turn on feature, but it is unable to be controlled via an automation system or IR.
The Rialto 600 also has phoenix style speaker connections, where the Rialto 400 has standard speaker posts.