The VH1 series released a teaser featuring the 49-year-old actress and artist Frances Bean Cobain, who will serve as guest judges on Friday's episode.
Pompeo and Cobain will help judges RuPaul, Michelle Visage, Carson Kressley and Ross Mathews judge the Best Judy competition. The contestants will take on a Judy Garland-inspired makeover.
"Tonight, @EllenPompeo and @The_SpaceWitch join the judges' panel for some Best Judy makeovers!" the show tweeted Friday. "See you on the main stage at 8/7c on @VH1!! #AllStars4."
RuPaul's Drag Race All Stars also released a preview of Friday's episode that showed the contestants react to Valentina's departure.
"We are mourning the loss of our dear departed Valentina," Latrice Royale says in the clip. "It's a little weird without her energy in the room and we miss her whole spirit of existence."