If Pettersson's still struggling with a knee problem, there is absolutely no reason for him to go to 4 Nations. He's doing very little with the team right now so they should just rest him until he can perform again. They can call up Raty.
But I'm really starting to think that this is just not the market for him. I found it interesting in the recent The Athletic player poll that players say they wouldn't want to play in Canada because of the scrutiny. It can't be easy to play literally one bad period and have the fans calling for your head. You'd like to see fans take it easier on players who are struggling, but I guess it is what it is.
It definitely leaves the team in a conundrum. If they can find a credible replacement, they should probably do so. But it will be hard. No, Dylan Cozens is not a credible replacement. There are no free agent centers that would be interesting. I don't see anything on the trade market.