Every historical precedence suggests that Karlsson will be in the Hall of Fame.
The Norris trophy was first awarded in 1954. 16 retired/eligible players have been a finalist 4+ times. Every single one of them has been inducted to the Hall (full list - Bourque, Lidstrom, Orr, Harvey, Park, Potvin, Robinson, Chelios, Coffey, Pilote, MacInnis, Gadsby, Kelly, Pronger, Salming, Horton).
Karlsson is one of several active players with 4+ years as a Norris finalist (along with Chara, Hedman, and Doughty). There shouldn't be any doubt that all of them will get in.
Even with three years as a Norris finalist (ie one fewer than Karlsson has) almost always gets you into the Hall. Eight eligible defenseman are in (Stevens, Leetch, Howe, Niedermayer, Langway, Blake, Pronovost, Flaman). The only eligible player not inducted is Bill White. (And Karlsson has won two Norris trophies and was runner-up twice; White had three third-place finishes. Karlsson has already passed him in games played, and White never had a playoff run as strong as Karlsson's 2017). It's virtually certain Karlsson will get inducted.