So just to make sure I am understanding correctly you want to use 4 different WAN(Public) addresses with 1 assigned to your NAS. I may be miss understanding but heres something I have done in the past
Interface
eth0 – WAN e.g. 10.0.1.1 - 10.0.1.2 - 10.0.1.3 - 10.0.1.4
eth1 – LAN1 192.168.1.1/24
NAT Translation
Source NAT Rule
Source =192.168.1.10(NAS private IP) > Destination =10.0.1.2 (Public IP)
Source masq eth0
Destination NAT Rule
Destination =10.0.1.2 > Translation =192.168.1.10