He sits down carefully, and it's not long after they're seated that Frisk is... touching him. It's really quite nice. They're warm and comfortable, and the pressure in S-4's side is just a little constant reminder that he isn't alone.

S-4 is actually a little bit warm as well. Even if he used most of his magic healing Frisk, he unconsciously uses a bit of it when he gets close to others in need of support. He's only ever used it on Sans, but it should still be slightly soothing and cozy for Frisk.

Of course, if that doesn't help, S-4 wrapping one arm around Frisk and letting them snuggle closer might do the trick.