You have 4 different WAN gateways? That is do say, you have 4 WAN subnets, right?
So add your subnets to WAN interface in Firewall > Virtual IPs.
Than add the 4 gateways in System > Routing.
After that you can go to Firewall > NAT > Outbound, mark "Manual Outbound NAT rule generetion" and click the save button. Configure the outbound NAT rules for each of your servers to use the gateway you want it to use.
Now it's should be done.