What is the best way to cut memory foam?

I have a queen size bed, and i accidentally bought a king size memory foam topper. It is 4 inches thick. Once I got it out of the box and realized it was the wrong size, we couldn’t get it back in to send it back. So we need to trim it down. What would be the best way to cut it?

Either a large pair of sissors or an electric kitchen knife. Probably the second is the best unless you happen to have a really big pair of sissors.