The series tells the story of killer and rapist Stephen Port, who drugged and raped four men in his home in Barking, east London.
Port used the gay dating and hookup app Grindr to target young men and lure them to his flat.
The series stars Sheridan Smith as Sarah Sak, the mother of Anthony (played by Tim Preston), a 23-year-old fashion student originally from Hull who was killed by Port.
Viewers noticed that Shaun Thomas, the actor who plays Anthony’s brother, Paul, was in Emmerdale.
Thomas starred in the popular ITV soap opera as Gerry Roberts between 2017 and 2018.
The character appeared in 86 episodes and was a cellmate at Skipdale Young Offenders Institute where he was awaiting trial for the attempted murder of his grandfather.
Emmerdale fans rushed to Twitter after recognising Thomas. One person tweeted: “The actor playing the brother is fantastic - he was in #emmerdale right? #FourLives.”
While a another fan tweeted: “GERRY! #emmerdale #fourlives.”
The series has received lots of praise online with fans calling Smith's performance "absolutely incredible".
"Once again @Sheridansmith1 is absolutely incredible,” one viewer on Twitter said.
They added: “Such a powerful, emotional performance. A difficult, but important watch #FourLives.”
Another viewer tweeted: “Sheridan Smith is a brilliant actress #FourLives.”
While a third shared: “Sheridan Smith is an amazing actress there isn’t a role she can’t play #FourLives.”
Stephen Merchant, who is typically known for starring in comedies, plays the ‘Grindr killer’ Stephen Port.
Speaking about playing Port, Merchant told the Telegraph that he was “really unsettled, really upset” to learn how long the police took to arrest Port.
“When I first read the script, I wanted to believe that some of the many police errors in the investigation must have been overstated for dramatic purposes. But they weren’t, were they?” He said.
The final episode of Four Lives airs tonight at 9pm on BBC One.