Kate Middleton's solo visit to the headquarters of the mental health service Shout on Wednesday, Jan. 26, is the final nail in the coffin to the Fab Four. In fact, a royal biographer believes the union between the Duchess of Cambridge and Prince William and Meghan Markle and Prince Harry has long ended.

Kate Middleton visited Shout to meet its volunteers alone. It has been known that the text service was set up by the Fab Four, composed of her, Prince William, Prince Harry and Meghan Markle, in 2019.

So what happened after that? The relationship between these two couples has been marred with different issues and controversies since the Sussexes stepped down as senior royals at the beginning of 2020.

With that being said, royal biographer Robert Jobson said the visit only highlighted the "irreparable" relationship between the four, Express noted. He believed it could have been a "Fab Four" day out if only things were different today.

Sadly, for Jobson, "that ship has long sailed." Shout happens to be the first major project of Kate Middleton, Prince William, Prince Harry and Meghan Markle together. They founded it in 2019, knowing mental health is a subject they were all extremely passionate about.

However, Jobson revealed that there were already tensions happening behind the scenes when the two parties launched the service. He told The Mirror that long before the issues brewing between them went public, they had decided to use £3 million hard cash from their then joint Foundation.

Despite that, though, people who were close to them already knew the "discontent simmering below the surface." A lot has happened since then and it was all beyond repair.

Prince Harry and Meghan Markle accused the royal family for being racist in an interview with Oprah Winfrey, though they chose not to name them. The Duchess of Sussex also revealed being "a working royal made her feel suicidal."

Jobson said Kate Middleton perfectly handled her engagement regardless of these issues, although she was alone.

Meanwhile, talking about Shout, the platform has seen an increase in its usage by a whopping 140% since the COVID-19 pandemic began. So, the future queen consort learned its need for more volunteers to meet the growing demand.

Kate Middleton was also told that the talks taking place on the text service could be used to provide insights on mental health in the U.K. It was revealed that suicide, depression and anxiety or stress were the main reasons texters sought help.