My parents live in Dallas and they have come to the airport to spend time with me at one of the Hyatt lobby restaurants a couple of times now. This is with a 3 hour layover. They are both fairly nice restaurants and a great place to catch up.
However, 4 hours is enough time to actually leave the airport, too. If they are just connecting, then they don't have to worry about checking in and can show up at the airport 45 minutes before their flight (assuming they didn't loose their boarding pass for the next flight). That gives you over 3 hours to spend together outside the airport. As for suggestions on where to go, that I cannot give.