The result left the defending champions level on points with forthcoming opponents Shakhtar Donetsk at the top of Group E, but the visitors had to weather several uncomfortable moments at Copenhagen's Parken Stadium.
Had Petr Cech not clawed Joshua John's shot onto the post in the 73rd minute, Chelsea would have seen the advantage procured by Juan Mata's 33rd-minute opener cancelled out by their enterprising opponents.
However, Cech produced what Di Matteo described as "a crucial save", and further goals from David Luiz, Mata and Ramires added a slightly misleading gloss to what had been a far from fluent performance.
"We expected a difficult game and that's exactly how it turned out to be," said the Italian, who led Chelsea to a penalty shoot-out victory over Bayern Munich in last season's final.
"We were a bit slow out of the blocks in the first half, but we still had chances to score, and we scored one goal.
"The second goal was important for us, to settle us down. It didn't come in the first half, but in the second half we were much sharper and once we'd scored the second goal, we knew we were going to win the game."
On a night when Bayern Munich were stunned 3-1 by Belarusians BATE Borisov and Manchester United had to come from behind to win at CFR Cluj, Di Matteo said Nordsjaelland's spirited showing merely illustrated the strength in depth in the competition.
"They tried to play their football and it wasn't easy," he said.
"You look at the Champions League results tonight (Tuesday) and there are no pushovers. Bayern lost, United had to come back. The Champions League is difficult.
"It just shows what I thought about the group. Shakhtar are the Ukrainian champions, and a very good team. It's going to be hard for us to qualify."
The quality of Nordsjaelland's football prompted Di Matteo to predict a "bright future" for the club, but his opposite number, Kasper Hjulmand, was left to rue the Danish champions' capitulation in the final stages.
"Four-nil looks stupid on the scoreboard, but I think we played a tight game for 75 minutes," he said.
"We collapsed with the second goal. Just before we had that good shot (from John), which was a good opportunity. Before that, we played well, we got the chances, but in the last 15 minutes we didn't do well."
The competition debutants lost 2-0 at Shakhtar in their group opener and with a home game against Juventus now looming on the horizon, Hjulmand says his side must adapt quickly to the physical demands of the Champions League.
"Some of it is definitely fatigue," he said of his team's late collapse.
"I had the same feeling in Donetsk. Physically, we're still lacking some fitness. The English Premier League, compared to the Danish league, is different.
"It's difficult to get the right physical condition to play these matches. That's one part of it. Another part of it is that we didn't score.
"It would have been fun to see 1-1 at 73 minutes and see what happened, but of course Chelsea have great quality, and they deserved the win at the end."
Source: AFP
Source: AFP