"Obi-Wan Kenobi" Episode 4 begins with a frantic montage of the eponymous Jedi being treated for his burns — wounds sustained in his duel with Darth Vader in Episode 3. Tala manages to get Obi-Wan out and escorts him to a rebel base used for rescuing Force-sensitive fugitives. There, he's submerged in a bacta tank so that his body can properly heal.
While in the bacta tank, Obi-Wan has flashes of Anakin, seeing his old padawan's severed limbs and terrible scarring as his own. It's possible that this is simply a nightmare caused by Obi-Wan's trauma, and that the specific shots of Anakin in his own tank are more for the sake of the audience than they are actual reflections of what Obi-Wan is seeing. However, it's also possible that, now that the two have encountered one another again, Obi-Wan is seeing flashes of Anakin through the Force.
There's precedent for that kind of connection, be it between Luke and Leia in the original "Star Wars" trilogy or between Ahsoka and Vader in "Star Wars Rebels." However, those connections have typically been more feelings than visions, and they often relied on physical proximity. Perhaps the two former friends' shared experience strengthened their connection through the Force in some way, causing Obi-Wan to have vivid visions of Anakin's pain. Or, maybe, it was just a nightmare.
