Gilgo Beach serial killer Rex Heuermann unleashed a four-day murder ritual on his unsuspecting victims — ending with him meticulously trying to cover up his gruesome crimes so he could keep his double life a secret, a therapist revealed.

The bloodthirsty killer, who spent his days with the veneer of a normal suburban family man, would befriend his victims, kill them, dump their remains and then try to discard evidence, therapist Alison Winter reveals in the fourth installment of the Peacock doc “The Gilgo Beach Killer: House of Secrets,” which was produced in partnership with The Post.

“It’s a four-day high. A four-day adrenaline rush and then he would [meet] his family,” she said in the episode, which premiered Thursday.

The hulking architect sadistically planned how he was going to kill and torture his sex worker victims at his Massapequa Park home – before reuniting with his unsuspecting family, Winter said.

Heuermann first made sure his basement was clean before murdering his victims, according to Winter — who said he fessed up to the crimes at a jailhouse meeting last summer.

Then, he would engage in what she described as “playtime” before dumping their remains – something which he viewed as a sadistic game, according to the doc.

“He would hit the timer, dump the body, get back in the truck and hit the timer again,” she said.

“It became more of a game inside his mind. Prepping, timing, playtime, cleanup. It all became a methodical second life.”

It took Heuermann two minutes 32 seconds to dump his first victim, identified as Sandra Costilla in 1993 — but got the time down to just 37 seconds by the time he killed his final victim, according to Sayville-based therapist Winter.

“It [Killing] became an outlet. It became an obsession,” she said, noting how he was inspired by the violent material he read.

On the third day, Heuermann then spent time covering up the murder scene – tidying up the basement and getting rid of any tools.

Heuermann kept the fourth day clear for so-called “emergencies” before reuniting with his family.

Winter revealed how Heuermann was able to juggle his killing spree with family life, commenting on how he murdered Karen Vergata – a crime he was never formally charged over – just days before he tied the knot with Ellerup in Sweden in 1996.

The killer said he stopped murdering in September 2010 because he didn’t get the same pleasure from it.

But, John Douglas – a FBI profiler – met with Winter and described Heuermann as a “malignant, narcissistic, sadistic, psychopathic serial killer.”

“Had he not been apprehended, I think he would have killed more,” he said.

Suffolk County prosecutors previously revealed how Heuermann kept a dossier, where he mapped out his killings and shared his desire to keep on killing.

Heuermann kept a checklist, instructing him to get rid of wipes, towels, items he had worn, and any evidence that could connect him to the crimes.

His notes contained sections titled “Pre-Prep,” “Prep” and “Post-Event.”

Heuermann – who had no criminal record before being charged – sensationally changed his plea in April.

He pleaded guilty to killing Amber Lynn Costello, 27, Megan Waterman, 22, Melissa Barthelemy, 24, and Maureen Brainard-Barnes, 25, who were famously known as the “Gilgo Four.”

He also confessed to the murders of Valerie Mack, 24, Jessica Taylor, 20, and Costilla, 28.

Heuermann will not be charged over Vergata’s murder as part of his plea deal.