Although I never book these kinds of fares (hate long layovers, even when spent in a hotel room), let me offer my opinion on what the next available flight means. This question was specifically asked by the OP in the above referenced thread. IMO the first flight with any seat open will count as the first available flight - doesn't have to be in your desired cabin. You may be finding all G fares up to your layover (forced kind-of-a-stopover) point but then the first available flight may only have F available. If there are no other flights with coach availability within 4 hours on a domestic itinerary you will have to book the F fare (at most likely very high price) or not be able to fly on your desired flights with G fares.