IPv4 Address

How do I keep my IPv4 address when switching to the newest XFi modem?  I have an XB6 now and returned an XB7 a few years ago because it came with an IPv6 address and no one at support could help me.  I have just receive the new XB8 and want to make sure I keep my IPV4 address before I go through the hassle of switching out the modems.