No, ispconfig does not handle ip routing as you describe.
Like till said;
You will have to have four public ip addreses
Each server will have it's own private ip address
You should have a router that will handle the forwarding of each public ip to each private ip.
On installation of the os etc onto each machine you should refer to it by it's own private ip. But when you want to make connections to it, via the public ip. (don't quote me on that last line!)