It’s 5/4 x 3 wood. On the 3 ft there are 2 stringers. On the 4ft there are 3 stringers. Both have old wood on them and the 3 ft is significantly better than the 4 ft there is almost no sag to it with older wood. The 4 ft has a lot of sag but the wood is in worse shape. I would use 5/4 x 6 treated pine for the new slabs.