1. Take your four pallets of the same size and stack them on top of each other two by two. No need to secure them, the weight of the wood holds them in place just fine. 2. Make sure your one remaining pallet is wide enough to fit as a back rest. In this case the top boards need to be long enough to fit as a back rest while still accommodating the 2X4s on each side of the seat. So they need to be the width of the seat plus at least 4 more inches.