Jackett said: "The result pleases me a lot. It was a tight game for quite a long time and I couldn't necessarily see the 4-0 coming.
"The second goal was a very good goal from our point of view after we'd been under a bit of pressure and I think that's what killed the game off. I couldn't see us losing once we'd got that second.
"I thought Darius Henderson was terrific up front. He hasn't got on the scoresheet himself but his link-up play with Chris Wood was really good."
Millwall moved to 15th following the win over the Terriers.