I know the 4th gen 4R and GX470 are the same part numbers for skid plate and tank. Same with the 5th Gen 4R and GX460. Fuel capacity of the four is same. Both sit in the same location with each using six bolts for the skid plate. I've seen Toyota change part numbers on same part. Either used in a different vehicle or something related to a part changes so the part got a new number as well. I was under the 4R making sure the transfer case oil was okay and noticed how close the fuel skid plates looked.
I am in AZ and rust is a non issue. I've debated mounting the full skid plate and spacing the half skid plate on the leading down just enough to fit. Zero value in half skid plate and leading edge is the area that most likely going to be hit. Plastic tank is relatively cheap but no fun being miles from pavement with hole in the fuel tank.