SNK-P0040AP4 is modified from SNK-P0035AP4. The major difference is the location of the heat sink fan. For SNK-P0035AP4, the heat sink fan is at the front side of the heat sink body. So, the cooling air is pushed into the heat sink fins. For SNK-P0040AP4, the heat sink fan is at the rear side of the heat sink body. So, the cooling air is sucked through the heat sink fins. The advantage of SNK-P0040AP4 is that the rear located heat sink fan can help to cool down rear CPU memory VRM components especially for X8 DP workstation motherboards.