That remains Calvisano's only home win in the Heineken Cup and Williams is desperate to avoid more humiliation with the Ospreys tomorrow.
Williams said: "It was a massive shock to lose, no one had even heard of Calvisano. We let ourselves down with our mindset, we went out there expecting to win and were leading by about 20 points at half-time.
"Then we just switched off and they made us pay. It proved you cannot underestimate anyone in this competition. They have earned their place in the Heineken Cup and we won't make the same mistake again."
Gavin Henson lines up at fly half, inside James Hook, while Jonathan Thomas has recovered from a shoulder injury to start.
Coach Lyn Jones, who led Neath on that fateful day four years ago, said: "Calvisano are already out of the competition but they are playing for pride. We would be a big scalp for them. I've learned from experience that you're in trouble if you underestimate any opposition.
"They have a French mentality that influences the way they play. They have a big set piece so we must not stop playing rugby against them."