Hello and welcome to the forum! Friends & Family is available on Nationwide Single Line plans with 900 Anytime minutes or more, as well as Nationwide Family SharePlans with 1400 Anytime minutes or more. Is the fourth line of yours under one of these plans as well? It may not be. Other than that I see no restriction that would prevent that fourth line from qualifying for friends and family unless there is some kind of pending account issue with that particular fourth line.
WoW, in that case I don't know either. You certainly seem eligible and I searched the FAQ's for some other restriction that might come into play for you and I could find none. I would give them a call and ask. Please post back and let us know as there's nothing in their information that restricts Friends & Family to only three lines.