What if he wants a DMZ with his external address for web servers or what ever else. He wouldn't need to NAT to all 5 of his external addresses. He'd need to NAT to one and use the other 4 for external facing devices. I think we need more specific's to answer any questions.
"I can picture a world without war. A world without hate. A world without fear. And I can picture us attacking that world, because they'd never expect it."
- Jack Handey, Deep Thoughts