The new RB604 is an expansion for RouterBOARD 600 and also the RouterBOARD 500 series. It attaches to the RB device to add 4 additional miniPCI slots, your RouterBOARD 600 can now have 8 miniPCI slots total!
The RouterBOARD 600 is the high performance wireless platform. It has four miniPCI slots and three gigabit ethernet ports and it is the fastest wireless board that MikroTik has ever made.