The dsl interface will get just one ip address assigned to it from isp. But since isp has allocated 5 ip address for that connection, you can use rest 4 public ip address for NAT pool and map those 4 ip address with the internal ip address like web server, email server, vpn server.
When isp gives more then one ip address (a subnet) for a connection, it will install a static route for the subnet allocated to that connection with a default gateway.
So lets say you will be given 1.1.1.1 to 1.1.1.6 with subnet mask /29 and if it allocates 1.1.1.1 to dsl interface. ISP will install a route like this
ip route 1.1.1.0 255.255.255.248 1.1.1.1
Where for rest 1.1.1.2 to .6 the default gateway is 1.1.1.1
So all the packets to .2 to .6 will be forwarded to 1.1.1.1.
So you can have rest .2 to .6 for global outside ip nat pool which you can statically map them to internal ip address for web,email,vpn server etc.
Hope this explains.