I like the tandem axle (a.k.a. "walking beam" or walking arm") suspension. I have an ATV trailer with that, and it's also on my Metavic Forwarding trailer. It certainly makes a difference when pulling a loaded trailer over uneven terrain. However, I have never seen one that is also street legal. Maybe the fender requirement is an issue? Fitting fenders around them is tough.
I'm not such a fan of the 4 wheel carts in my woods. The way they corner can make them tougher to maneuver on woods trails. They do work well in more open spaces (I don't have a whole lot of open spaces, however.)