S-4 doesn't really seem that surprised by the hug, but... Frisk's words are definitely enough to shock him. He freezes halfway through returning the hug, instead bringing his hands up to nervously fiddle with them.

"...Do you really?"

Love is something very important, from what he's seen, and something not easily given away. He can't remember anyone ever saying that to him. He's not even sure if Sans or Sans-Serif has said it out loud.