+1 on the two large diameter wheeled cart concept. They are a lot easier to move on rough terrain because of the large wheels. There's a reason that many donkey carts in the third world are of a similar design. Over smooth roads the 4 wheel version takes less energy (you're not supporting the weight of the cart as much) but if I have to drag a load over field and dale (and I have) I'll take the 2 wheel cart.