I'm currently in a relationship with a married couple. I started by dating him, and grew closer to her. Eventually, all three of us are dating. (Full disclosure: we've only been dating about 3 months).
Interestingly, recently he admitted that they considered a quad the ideal relationship. After reading about all the couples looking for a unicorn, I was a bit surprised. They have been poly for almost two decades, so they've had triads, V's, etc.
In time, after this relationship has developed a foundation, I can imagine adding another man... completing a quad. I've discovered that being open to whatever comes---well, amazing things happen.