I don't know much about quads. I did look up some stuff for triads. The general lore is that they tend to be less stable than vees. I think it has something to do with people changing in relationships. Vees are more flexible than triads. So I think a successful triad is one in which people try to maintain flexibility in their relationship. Let the love and relationship grow stronger or weaker as it is needed.
So my guess is that a quad may be similar. I think if the people can be flexible, it has a greater chance of success.