Next is not a service plan. It is only a plan for buying a new phone. So, if you do decide to buy 4 new phones on the Next plan and each one of those new phones is $20/month, then yes, you would be paying $160 + 4 x $20. If you only buy 1 new phone, then it is $160 + $20.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.