Game 4 – down, 2-1, in the series.

OK, not a “must win,” but pretty close. After 82 games and three in the postseason, the Boston Bruins’ backs are, well, not against the wall. But that wall in full view Wednesday night at TD Garden.

Some good news with the return of defenseman Colin Miller. The defense still depleted with Torey Krug, Brandon Carlo and Adam McQuaid on the mend.

Aside from the emergence of Charlie McAvoy as a bona fide NHL blueliner and the stellar play of captain Zdeno Chara, what’s the best under-the-radar story of the Black-and-Gold in April? The play of Kevan Miller. He hasn’t registered a point in the playoffs, but his top-end minutes and lights-out defense, especially in the physicality part, have helped to stabilize a fractured back end.

“He’s been real solid for us,” Bruce Cassidy said about Miller pregame. “And he needs to continue to be real solid.”

Along with a cast of 22 others, starting in the 24 square feet. Tuukka Rask, giving up eight goals in Game 2 and 3.

Want two more stats that were front and center before the 7:30 puck drop?

Six of the Bruins’ eight goals this series have been scored in the second period. They have outscored the Senators 6-3 in second periods, but have been outscored 2-0 by the Senators in the first period.

Home teams win only 49 percent in Game 4 of a seven-game series in Stanley Cup Playoff history. That is the worst home-ice advantage of any game.

The feel-good prelim info? The League announcement that Patrice Bergeron has been named a finalist for the 2017 Frank J. Selke Trophy, given annually to the forward who best excels in the defensive aspects of the game. This is the sixth straight season Bergeron has been named a Selke finalist, winning the award in 2012, 2014 and 2015.

“Every game is a new game,” Bruce Cassidy said pregame in an effort to downplay the significance of the upcoming 60 minutes. “I think every coach will tinker a little bit if things don’t go their way. But, the approach doesn’t change much. We need to win four before they do. Limit their chances and play on our toes and get this thing turned around.”

Here’s what we learned with the Bruins now in “must win” mode as the series shifts to Game 5, Friday night in Ottawa.

Four games – four first periods – 0 Black-and-Gold goals

Not for lack of opportunities in Game 4. Monday night just three shots on net in the first 20; Wednesday, twelve.

The monkey is getting bigger on the back of No. 63 since his Game 1 heroics. Brad Marchand in alone on Craig Anderson at the three-minute mark. The weak backhand dribbler goes for naught. Two minutes left, Marchand again on the breakaway with Anderson on the poke check to deny a first-period lead.

“I had two grade-A chances,” Marchand said, “and should have capitalized on at least one of them.”

Seconds left, and Ryan Spooner, Frank Vatrano and Drew Stafford break in 3-on-1. Spooner’s wrister a routine save for Anderson.

“We did have our ice and we did have our opportunities,” Cassidy lamented. “We weren’t able to bury them.”

The Sens had their own good bids. At 7:30 Eric Karlsson on Rask dead to rights, followed by Mike Hoffman’s effort with Colin Miller draped on his backside at 11:09 that gave Ottawa the period’s only power play.

McAvoy (almost) gets his first NHL goal

At 10:49 of the second 20 minutes, Charlie McAvoy sent a seeing-eye 60-footer from the right point that found twine for his first NHL goal, and a (short-lived) 1-0 lead. Noel Acciari clearly offside on the Guy Boucher challenge to keep the game scoreless after two periods.

“I think the Boston Bruins fans are seeing something right now that they’re going to truly appreciate for years,” Cassidy said about his rookie defenseman who logged a Boston tops 25:03 of ice time.

Karlsson and Ryan (again) in the third for game winner

Ottawa’s two leading scorers teamed up for the game’s only goal at 5:49 of the final frame. Karlsson sent a shot intentionally wide of Rask. Bobby Ryan, sitting on the doorstep for the dasher rebound and an easy poke around Rask for the 1-0 lead.

“He’s making great plays, but it’s way beyond that,” Ottawa coach Guy Boucher said about Karlsson, the best player in the series thus far. “This guy has become something else, in every aspect.”

Indeed.

Combined, Karlsson and Ryan now have 10 points in the series. One less than the combined total of Bergeron, David Backes, Spooner, Marchand and David Pastrnak.

“Inside our locker room, we need a little bit more from our offensive guys,” Cassidy said in understatement about his anemic offense.

“He’s a good player,” he followed about Ryan. “We’ve seen that in this series. He’s a big body and that ends up being the difference in the game.”

At 7:30 Ryan Dzingel dings the post after splitting Boston’s D to keep life in TD.

But the Senators held Boston to five shots on net – none over a 12-minute span – and two in the final two minutes with Rask pulled.

Friday night in Ottawa

“It’s not over until it’s obviously over,” Chara said. “We know it’s still playable, it’s 3-1, it’s not an ideal position but we know that we can play the same way, even better and get better results.”

“There are some guys that are just missing the net more often,” Cassidy said, “and it doesn’t allow you to get those second chances and stay inside and get those second chances.”

There won’t be any more chances of any number if the Black and Gold don’t get better results Friday night.