Four weeks ago, it seemed impossible that this feel-good Islanders season would end so soon.

The Islanders had been in a playoff spot since early December. They were buyers at the trade deadline. They were not quite a Cup contender, but with Matthew Schaefer driving the bus, the season’s vibe was impeccable.

Getting eliminated from playoff contention before the last game of the season even took place? Unfathomable, until it became all-but-inevitable on Saturday night. Impossible to digest until it became official, leaving no choice, on Sunday.

The Islanders’ 4-1 loss to Montreal formalized what has been an astonishing, gutting and indicting collapse over the last 25 days. Since losing to Ottawa and falling out of a spot on March 19, the Islanders are 4-9-0 in their last 13 games, a .307 points percentage that would rank 32nd in the league by a wide margin over the season.

Since the Devils moved to New Jersey in 1982, this is the first time that all three Tri-State Area teams have failed to make the playoffs in a season.

Had they merely been passably bad as opposed to awful, the Islanders would almost certainly have at least made the playoffs. Had they been average, the Islanders might have home ice in the first round.

Instead, in game after game, the Islanders fell flat. Two nights after that loss in Ottawa, the Islanders fell to pieces in the third period in Montreal. They came out as if it was a preseason game against the Blackhawks, got blown out 8-3 to the Penguins and — in the final blow for coach Patrick Roy — could not muster any desperation or energy whatsoever in must-win games against the Flyers and Hurricanes last weekend.

Then, with Pete DeBoer behind the bench and the Islanders knowing they needed to run the table just to have a shot, an 0-for-5 power play and 3-0 loss to Ottawa on Saturday put their playoff chances into Lloyd Christmas territory.

Reverse just one or two of those results, and the Islanders might be looking forward to the postseason now.

The details of Sunday’s match are, largely, immaterial. The Islanders fell apart during a 55-second span in the second period over which they gave up three goals, with Nick Suzuki, Ivan Demidov and Alex Newhook doing the scoring. Offensively, they failed to threaten through two periods and failed to build Casey Cizikas’ third-period goal into anything more than a brief spurt of momentum. Zach Bolduc added a fourth for Montreal.

There is still one game left, a now-meaningless bout with Carolina on Tuesday that could get an injection of excitement if Victor Eklund is brought up to make his NHL debut. Otherwise, the focus shifts to a hugely consequential summer.

General manager Mathieu Darche, whose midseason moves to bring in Ondrej Palat, Brayden Schenn and Carson Soucy did not get the Islanders over the playoff cutline, already set the tone for the next few months by swapping Roy for DeBoer with four games left in the season.

The immediate question now is whether captain Anders Lee’s last game as an Islander will be Tuesday.

The bigger-picture question is exactly how much of an overhaul Darche looks for over the summer, and what moves are possible given an underwhelming free agent class.

After two straight seasons without playoff hockey, it’s pivotal to find the right answer.