Create the relevant VLAN interfaces on Ether 4.
2, Create the relevant DHCP server, Network, IP Pool and IP Address entries for the two VLAN interfaces.
The easiest way to achieve the ISP selection is to create 2 VRF entries and place VLAN1 & Ether1 in one VRF and VLAN2 & Ether2 in the other.
Good. If you look in the routing table you should see routing entries for the various interfaces each marked with the routing-mark corresponding to the VRFs that you created.
If you add a static IP to an interface in a VRF then RouterOS should add the connected route into the correct routing table, but when you add the default route to the routing table you will have to manually select the routing-mark entry when entering the default route. Check your routing table with particular attention to the routing mark column.