You will need to assign the additional IPv4 addresses to your server after you've ordered them in the orderpage. This article will guide you step by step for adding ipv4 addresses to your dedicated server.
Go to https://my.snel.com/clientarea/services/
Click on the service for which you would like to manage the IP addresses:
Go to "IP Management" and click on "Add New IP"
Assign the new IP address a corresponding reverse DNS entry and click on "Submit"