Boudreau-Gagnon, who hails from Riviere-du-Loup, Que., and Marcotte, a Quebec City native, scored a combined 189.120 between their technical and free routines.
The Russian pair of Natalia Ishchenko and Svetlana Romashina — which hasn't lost an Olympic synchro event since the 1996 Atlanta Games — nabbed gold with 197.100 points while Spain's Ona Carbonell and Andrea Fuentes took the silver with 192.900 points. The Chinese duo of Huang Xuechen and Liu Ou won bronze.
Boudreau-Gagnon wasn't thrilled with the Canadians' result — despite improving on the sixth-place finish she and former partner Isabelle Rampling put up in Beijing in 2008. She added they could learn from it heading forward.
"For sure we are not happy about the result," Boudreau-Gagnon said. "But we just did two solid performances in a row and this will give confidence to everyone for the team event."
Marcotte echoed the sentiment, saying they still have the team event to look forward to which begins on Thursday.
"In the end, this is the best ranking in the duet event at the Olympic Games since 1992," she said. "We are here to perform and we are pretty satisfied with what we can control. Our focus is now on the team events."
The Canadian pair scored 47.180 for technical merit and 47.440 for artistic impression for a total of 94.620 in the free routine. They were fourth after Sunday's technical routine with 94.750 points.
Canada has not won an Olympic synchro medal since the 2000 Summer Games in Sydney, Australia.
Despite their domination in the sport, the Russians still felt they needed to earn every point.
"Of course we were under pressure, but we tried not to read the newspapers or listen to the radio," Ishchenko said. "People were trying to hang the gold medals around our necks days before. We just tried our best to focus on what we needed to do."