Taking to Twitter, Michele posted a throwback Polaroid-style picture of the two together.
She captioned it, "Hard to believe it's been 4 years... We miss you C... love you more."
Michele and Monteith first started working together on Glee in 2009, and then news outlets began reporting in 2012 that they were dating.
The couple had only been together about a year and a half when he died at the Fairmont Pacific Rim hotel in Vancouver from "a mixed drug toxicity" which reportedly consisted of heroin and alcohol.
After Glee ended, Michele followed the show's creator Ryan Murphy over to star in his next FOX hit, the horror-comedy series Scream Queens.
She's been there for the past couple of years but has now shifted most of her focus back to her music career.
Earlier this year, she released her sophomore album, Places, the follow-up to 2014's Louder.
The first single off the album is "Love is Alive," which she recently performed on The Late Late Show.
It's also been suggested that her new song "Hey You" was written about her relationship with Monteith.