The Bruins’ 4th line isn’t just exceeding expectations. They’ve been the best line in hockey.

“I think we’ve got to stop calling them the fourth line. I mean, it’s impressive — the way they’ve been playing.”

After yet another game in which his checking-line trio of Mark Kastelic, Johnny Beecher, and Cole Koepke tiled the ice in Boston’s favor, Jim Montgomery relented when asked about the Bruins’ fourth line.

“I think we’ve got to stop calling them the fourth line,” Montgomery admitted after Boston’s 5-3 win over the Avalanche. “I mean, it’s impressive — the way they’ve been playing.”

Montgomery’s point is valid, given the sizable — and admittedly unexpected — contributions that the Beecher-Kastelic-Koepke line has generated through five games.

One could make a compelling argument that they’ve been Boston’s top line so far in 2024-25, with the triumvirate combining for six goals and 15 total points out of the gate.

But a glimpse at the underlying numbers don’t just map out a bottom-six crew punching well above their weight on Boston’s roster.

Through five games, the Beecher-Kastelic-Koepke line has been the best three-man unit in hockey.

Just as we all predicted, of course.

“They’re earning all the ice time they’re getting, and their details, their work ethic, their puck support of one another, offensively and defensively, is allowing them to have tremendous success,” Montgomery noted.

Small sample size aside, the Kastelic line doesn’t boast the same star power as some of the other top trios in hockey.

They can’t put opposing skaters on their heels to the same degree as lines anchored by Nathan MacKinnon or Connor McDavid. They don’t boast the same two-way credentials present on top lines like the Kings (Anze Kopitar) or Panthers (Sasha Barkov).

But so far this season, Boston’s revamped fourth line has developed a knack for putting opponents on the ropes thanks to a seemingly ceaseless motor and a simple, north-south game.

And so far, the stats speak for themselves.

Through five games now, the Beecher-Kastelic-Koepke line has logged 40:04 of ice time together, per NaturalStatTrick. During that stretch, the Bruins have outscored opponents, 9-0.

Take away Beecher’s empty-net tally on Wednesday that iced Boston’s win in Denver, and that unit is still holding onto an 8-0 advantage in tallies during 5-on-5 play.

That lopsided offensive output when Boston’s fourth line is winning puck battles and throwing their weight around on the forecheck has been welcomed by Montgomery — especially as the rest of Boston’s revamped forward corps tries to find its footing.

Even with a second line featuring Charlie Coyle and Brad Marchand still looking for some cohesion (zero goals for, two goals against in their 41:25 of 5-on-5 reps), the Bruins are still landing punches thanks to their foot soldiers further down on the depth chart.

“That whole line’s been amazing for us. They’re the hardest worker out there, and they earn everything they get,” Hampus Lindholm said Wednesday.

What makes Boston’s sizable scoring advantage with Kastelic’s crew on the ice even more impressive is that the trio is seizing momentum while handed taxing defensive assignments.

While the Bruins have outscored teams, 2-0, during the 44:22 of 5-on-5 ice time that their top line of Pavel Zacha, David Pastrnak, and Elias Lindholm have been on the ice, they’ve also had 63 percent of their faceoffs set in the offensive zone.

The Kastelic line has only had 34.6 percent of its faceoffs set in the offensive zone during 5-on-5 action, with Montgomery usually tasking them with daunting D-zone reps, usually while matched up against top-six skaters.

Given those unfavorable shifts — usually spent treading water in their own zone — many effective fourth lines are not awarded with sterling stat lines.

The last time Boston went to the Stanley Cup Final in 2018-19, its fourth line of Chris Wagner, Sean Kuraly, and Noel Acciari was a key tone-setter for that roster. But over their 401:55 of 5-on-5 reps that year, Boston was actually outscored when they were on the ice, 14-8.

Given the limited number of starts they had in the offensive zone, it wasn’t necessarily expected for that grouping to finish the year with a positive goal differential.

It’s what makes the Kastelic’s line scorching start — with those eight 5-on-5 goals already equaling the Kuraly line’s season-long output in 2018-19 — even more surprising.

After recording 10 points (seven goals, three assists) over 52 games during his rookie season, Beecher is now up to five points in five games. Beyond his empty-netter Wednesday, Beecher provided the screen in front of Alexandar Georgiev that allowed Hampus Lindholm to blast a puck past the netminder at 9:22 in the second period.

Even with zero points on the scoresheet Wednesday, Kastelic left his fingerprints all over the rest of the box score — winning six of his eight faceoffs over 16:21 of ice time.

And Koepke — who entered the season with just three points in 26 NHL games over two years with the Lightning — is already up to two goals and five points in his first five games in a black-and-gold sweater.

He lit the lamp again Wednesday after tipping an Andrew Peeke shot past Georgiev at 13:46 in the first.

“Been a great surprise,” Montgomery said of Koepke, who landed four shots on goal in his 14:00 of ice time Wednesday. “I don’t think he’s been that much of a surprise to our pro scouts that recommended him. They spoke highly about him, and he’s rewarding those scouts and us with the effort and how he’s playing.”

It’s all but a given that Boston’s checking line is going to quiet down offensively as the year goes along.

At least, that’s the conventional thinking when it comes to fourth-line regulars, especially a Boston grouping with just 237 combined games of NHL experience.

But so far, this line seems to be establishing a track record of defying expectations.

Conor Ryan is a staff writer covering the Bruins, Celtics, Patriots, and Red Sox for Boston.com, a role he has held since 2023.