There is something more important to this win and that is the margin by which they won. If they end up being equal in points with PSG and they share rank 2 and 3, the direct comparison will decide which team will advance to the next stage in the Champions League and which team will be relegated into the Europa League. Winning 4-1 can be so important because PSG would have to beat Newcastle at least 3-0 to then have the goal difference decide about the order in the table. I think that's one reason why this defeat is really bad for PSG. It raises the pressure even more for the next games.
You are right. PSG lost by a big margin. And in this they may face problems. PSG lost the away match by 4-1. It is unlikely that PSG will win the home match by 3 goals. As I have seen Newcastle's gameplay, it is very difficult to beat this team by 3 goals. Because even when Newcastle play attacking, they never leave their defence vulnerable. The team has not been in good form in the Premier League at the start of this season. Even then, they conceded only 7 goals in 7 matches.
However, I think Newcastle and PSG are more likely to qualify for the knockout rounds. I had higher expectations from the Milan team. But their performance is a bit slow. They probably want to focus entirely on Serie A.
The next game for Newcastle will be key. If they win their home game against Dortmund, I would say that they have amost made it through the group stage unless they lose all of their last 3 games. I know there is still some speculation involved because the group is so close, but the team that has a head start in this close group is probably one of the teams that comes out successful. They have the goal advantage against PSG or they would have to get slaughtered. They have a home game against AC Milan and did not concede a goal there. They are in a great position to master the hardest group of this Champions League season.