It was a poor period of play that Pompey were unable to recover from at Northampton.
Ryan Watson scored twice in quick succession midway through the first half, while Fraser Horsfall and Sam Hoskins added further goals for the hosts before the break.
There was, at least, an improvement after the restart, with Ellis Harrison grabbing a consolation from the penalty spot following a foul on fellow substitute Harvey White.
But Kenny Jackett knows that by that stage, his side had left themselves with far too much work to do.
The Blues boss said: “Our defending was very poor, with the first three goals coming from a free-kick and then two headers from corners.
“And the fourth was from a ball over the top that we were in control of, but the wrong decision has been made.
“That gave us a mountain to climb and we’ve shot ourselves in the foot. That effectively ended the game.
"My focus is on where the players can improve – and obviously when you lose 4-1 there’s plenty of room for improvement."
“It’s frustrating because that’s something we’ve been very good at for a period of time. We got into a position where we left fighting for pride in the second half.
“We were able to get a goal back and we would have wanted more, but we understand Northampton were in control by then.
“We need to give ourselves a platform to build off if we want to have a chance to be able to go forward and do well.
“My focus is on where the players can improve – and obviously when you lose 4-1 there’s plenty of room for improvement.”
Jackett admitted that his squad now need an injection of belief to recover from their stuttering run of form.
He said: “The confidence of some of the players can get low. It’s hard-earned and can go quickly, but there’s only one way to get it back.
“That’s to roll up your sleeves and work even harder. That certainly has to be the case for us moving forwards.”