The polls closed at 9:30 p.m. ET (8:30 p.m. CT). CBCNews.ca will bring you the results as they filter in.

The four seats in question have been considered "safe" for the parties that currently hold them.

If any of these ridings changes hands, it would be considered a major upset — a departure from pattern that could shift the power dynamic in Ottawa.

Even if seats don't change hands, these byelections could serve as a barometer reading on how voters in four geographically diverse ridings perceive the current state of affairs and the leaders of Canada's two major political parties.

A Liberal flop in stronghold seats would suggest Prime Minister Justin Trudeau and his cabinet are in a tough spot. That would validate some national polls that say support for the government has slumped after nearly eight years in power.

A Conservative loss — or a failure to make inroads in potentially winnable seats — would suggest fledgling leader Pierre Poilievre still has work to do to win over voters.

Notre-Dame-de-Grâce—Westmount

The Montreal-area riding of Notre-Dame-de-Grâce—Westmount, an urban seat last occupied by former cabinet minister and astronaut Marc Garneau, has been Liberal red for generations.

While it's considered a Liberal stronghold, many anglophones in the riding were angered by the government's recent overhaul of the Official Languages Act.

There are 10 candidates on the ballot in the federal riding of Notre-Dame-de-Grâce-Westmount in Montreal. (CBC)

The Liberal candidate is Anna Gainey, the party's past president and a friend of Prime Minister Justin Trudeau. She's up against teacher Jean-François Filion for the NDP, student Laurence Massey for the Bloc Quebecois, accountant Mathew Kaminski for the Conservatives and the Green Party's deputy leader Jonathan Pedneault.

Winnipeg South Centre

Winnipeg South Centre, an urban seat held by Liberal MP Jim Carr until his death, has been in the Liberal win column for decades — except for a four-year gap period the 2011 election that produced a Conservative majority.

Ben Carr, a school principal and former political staffer, is running for the Liberals to replace his late father.

He's up against Damir Stipanovic, an air traffic controller running for the Conservatives; Julia Riddell, a clinical psychologist representing the NDP; Doug Hemmerling, an educator with the Green Party; Tylor Baer, a DJ with the People's Party of Canada; and Tait Palsson, running as an Independent.

Oxford

Oxford, a largely rural riding in southwestern Ontario, has been held by a Conservative since the Progressive Conservative and Canadian Alliance parties merged in the early 2000s.

Longtime Conservative MP Dave MacKenzie triggered the byelection when he stepped down in January.

His daughter Deb Tait ran for the party's nomination and lost — a result that prompted accusations by Tait of wrongdoing, which the party has denied.

Arpan Khanna, a lawyer who previously ran for the party in Brampton, Ont., will carry the Conservative banner.

In an unusual move, MacKenzie and his daughter have endorsed the Liberal candidate, David Hilderley, a retired principal and real estate agent, over Khanna, who has been called a "parachute candidate" for his tenuous connection to the riding.

Conservtive candidate Arpan Khanna, Liberal candidate David Hilderley and New Democratic Party candidate Cody Groat are running in the southern Ontario riding of Oxford, expected to be one of the night's tightest races. (arpankhanna.ca, davidhilderley.liberal.ca, oxfordndp.ca)

Speaking to the Canadian Press on Monday, MacKenzie said the party infighting has resulted in "the nastiest campaign that we've ever seen in our riding."

"It's divided our party and our community," he said.

Cody Groat, a New Democrat, is also on the ballot.

Portage-Lisgar

Manitoba's Portage-Lisgar, a riding long held by former interim Conservative leader Candice Bergen, is among the safest Conservative seats in the country — although the People's Party candidate posted strong numbers in the COVID-era 2021 federal election.

Conservative candidate Branden Leslie, a former political staffer who worked for a grain farmers' advocacy group, is being challenged by People's Party of Canada Leader Maxime Bernier.

Maxime Bernier, leader of the People's Party of Canada, speaks to reporters in Winnipeg on May 16, 2023 after appearing in court and being fined $2,000 for breaking COVID-19 restrictions in Manitoba in 2021. (Steve Lambert/Canadian Press)

Bernier is trying to exceed the 21.58 per cent his party got in the last election — the movement's best result nationwide.

Leslie and Bernier have traded barbs throughout the campaign. Bernier has called his opponent a "fake" conservative. Leslie, in turn, has called Bernier "an opportunist from Quebec who will say or do anything he thinks people want to hear."

To take on the far-right Bernier, Poilievre visited the riding and used rhetoric targeting the World Economic Forum — an international organization that has become a focus of many right-wing conspiracy theories online — during a stump speech.

Leslie and Bernier will be on the ballot alongside Liberal candidate Kerry Smith — who leads a Métis employment and training department — Lisa Tessier-Burch, a teacher running for the NDP, and Green Party candidate Nicolas Geddert, who has worked in housing.