Netflix's Riverdale. Both series are based on Archie Comics characters and were developed for television by Roberto Aguirre-Sacasa, so it makes sense that there would be some overlap.
The Chilling Adventures of Sabrina follows Sabrina Spellman (Kiernan Shipka), a teenage witch who, in season 4, must confront the cosmic horrors known as the Eldritch Terrors as they descend on her town of Greendale. Just across the river from Greendale lies Riverdale, where classic characters like Archie (K.J. Apa), Betty (Lili Reinhart), Veronica (Camila Mendes), and Jughead (Cole Sprouse) live. Even without magic and the occult, Riverdale often manages to out-weird The Chilling Adventures of Sabrina: its characters have dealt with serial killers, organ-harvesting cults, and deadly role-playing games.
The Chilling Adventures of Sabrina season 3 featured several references to Riverdale, including a trip to the town itself. No such trips occur in season 4, but there are still plenty of Easter eggs for fans of both shows to enjoy.
Riverdale's Sex-Ed
In the first episode of season 4, "The Eldritch Dark," Sabrina's best friend Roz (Jaz Sinclair) returns to her activist roots with a call for Greendale to have sexual education classes and proper resources. She wants it to be the latest assignment for the Women's Intersectional Cultural and Creative Association (WICCA) that she and Sabrina founded back in season 1. Roz knows that other schools in the area are much more progressive - she cites Riverdale High having condom machines in their bathrooms as an example. However, Roz's project quickly falls to the wayside as the Eldritch Terrors approach. This may only be a quick reference to Riverdale, but it helps establish these schools' proximities and knowledge of one another.
Sweetwater River
Chilling Adventures of Sabrina season 4, episode 3, "The Weird" features small Riverdale Easter eggs. The first is a reference to Sweetwater River, the river that separates Greendale from Riverdale. The Weird, one of the Eldritch Terrors summoned by Father Blackwood (Richard Coyle), arrives in Sabrina's life in somewhat of a Trojan Horse: a drowned man whose body has been brought to Spellman Mortuary. Where did this man drown? Sweetwater River.
Fans of Riverdale will immediately recognize the name Sweetwater River, not just for its geographic importance but for its plot relevance as well. Riverdale season 1 opened with Jason Blossom's body being found in Sweetwater River, a death which kicked off the entire plot of the show. Just like in Riverdale, a body from Sweetwater River brings nothing but chaos to the town of Greendale. Only this time, the villain is a mind-melding squid creature instead of an insane family of maple syrup moguls.
A Riverdale Transfer
"The Weird" introduces Lucas Hunt (Ben Ahler), a new student at Baxter High and Sabrina's lab partner. He and Sabrina bond quickly, even going on a date. However, Lucas gets scared when he kisses Sabrina and realizes that her tongue is becoming tentacle-like - a side effect of being possessed by the Weird. Things get even scarier for Lucas when he witnesses the Weird takes over Sabrina in biology lab and orders everyone in the class to kill themselves.
Naturally, after all this, Lucas leaves Baxter High and transfers to the other high school in the area: Riverdale. It's unlikely he'll find any semblance of normalcy there given the shenanigans that town gets into, but anything is better than the Eldritch Terrors. It's unlikely Lucas will feature in Riverdale given season 5's time jump, but his appearance would make for a fun Easter egg.
The Stoned Philosophers
In the most blatant reference to Riverdale throughout season 4, The Chilling Adventures of Sabrina sees three Riverdale actors come to Greendale and play very similar roles to their Riverdale counterparts. Sean Depner, Sarah Desjardins, and Doralynn Mui all appeared on Riverdale as Stonewall Prep students Bret, Donna, and Joan, respectively, who are all part of the Quill and Skull Society and antagonize Jughead for much of his time at Stonewall Prep. They even try to kill him in order to commit a perfect crime, but Jughead fakes his death and they are caught.
For their Chilling Adventures of Sabrina cameos, Depner, Desjardins, and Mui play of a band called "The Stoned Philosophers," which challenges Harvey (Ross Lynch), Theo (Lachlan Watson), and Roz's band to a Battle of the Bands. While Depner, Desjardins, and Mui are not playing their roles from Riverdale - Depner's character is named Tobey instead of Bret - they exude the same smugness and superiority as their Stonewall Prep characters. Their band's name is a clever reference to Stonewall Prep and its pretentious philosophies as well, fully drawing the connection between both trios. Tragically, the Stoned Philosophers are not in Greendale for long. They are brutally murdered by the undead band Satanic Panic and cannot compete in the Battle of the Bands.
The Fred Heads
It turns out that the Stoned Philosophers were not the first band that Satanic Panic attempted to battle. Baxter High's librarian shows Sabrina and her friends an old poster for Greendale's last Battle of the Bands. The two bands going up against each other are Satanic Panic and the Fred Heads. The Fred Heads was Fred Andrews's (Luke Perry), Archie father's, band. He started the Fred Heads along with Jughead's father, FP Jones (Skeet Ulrich), but he gave up his musical aspirations when his father ed away and he took over his construction business.
This quick nod to the Fred Heads is a sweet tribute to one of Riverdale's most beloved characters - and one of its few voices of reason - as well as to the late Luke Perry. It is also a reminder of how much Aguirre-Sacasa seems to enjoy incorporating musical numbers into both Riverdale and The Chilling Adventures of Sabrina, from cheerleading numbers and band practices to the ever-controversial musical episodes.