Yes, you can. The way to do is as follows.
(1) 2 Unleashed R510s on subnet-1 broadcasting SSID-1. One of the APs will act as Master.
(2) Other 2 Unleashed R510s on subnet-2 broadcasting SSID-2. Another Master AP for this group.
In essence, you are creating two Unleashed networks and separating them by VLAN / subnet so that they don't talk to each other.
Does this help achieve your goal?