Lupita Nyong'o is reflecting on grief four years after the death of her former co-star Chadwick Boseman.

The "Us" actress, 41, shared a black-and-white photo of the late "Black Panther" star Wednesday on Instagram to mark the fourth anniversary of his death. In the caption, Nyong'o, who plays Nakia in the "Black Panther" films, included a quote about grief attributed to an unknown source.

"Grief never ends," the caption read. "But it changes. It is a passage, not a place to stay. Grief is not a sign of weakness, nor a lack of faith. It's the price of love."

She added, "Remembering Chadwick Boseman. Forever." The post also included a photo of Nyong'o and Boseman smiling side-by-side.

Boseman, who starred as the title superhero in "Black Panther," died on Aug. 28, 2020, at age 43 after battling colon cancer for years. His cancer diagnosis was not publicly known before his death.

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The actor's passing sparked an outpouring of grief from his fans and co-stars, with Nyong'o saying at the time that she was "struggling to think and speak about my friend, Chadwick Boseman, in the past tense."

"It doesn't make sense," the actress wrote in 2020. "The news of his passing is a punch to my gut every morning. I am aware that we are all mortal, but you come across some people in life that possess an immortal energy, that seem like they have existed before, that are exactly where they are supposed to always be – here! … that seem ageless. Chadwick was one of those people."

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Earlier this year, Nyong'o starred as a woman dying of cancer in "A Quiet Place: Day One," and she told USA TODAY that the film helped her cope with the loss of Boseman.

"I have been in a season of thinking about mortality, and this film brought me even closer to almost seeking answers to the questions I've been asking with the loss of that friend of mine," she shared. "It was scary to have to live in that tenuous place making this film; I was afraid to go there. But once I was there, there's something about knowing you will surely leave this Earth that makes living much more worth doing."

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In a post on the third anniversary of Boseman's death in 2023, Nyong'o reflected that hearing the news of the loss was a "singular pain" and that the "confusion was so profound that it took months to trust the feeling of joy again."

Contributing: Patrick Ryan