There are different approaches - you could route between subnets on mikrotik2-4 and have static routing rules on mikrotik1 so traffic is directed to the correct mikrotik, or you could use mikrotik2-4 as switches with VLANs and perform all of the routing/firewalling on mikrotik1 which is probably the more common method. http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1 is a good starting point.