If the meta slows down significantly, Arfus can see play. He's a slow card but not a weak one. The deathknight cards are just insane in terms of relative power.
The reason he's so understatted compared to The Lich King is because having access to such powerful cards earlier in the game can make all the difference in the world. A 3 mana Diet-Flamestrike before Turn 8+ or Army of the Dead on Turn 6 could just outright win you the game. These things aren't (generally) possible with The Lich King...
This will be better than The Lich King most of the time. Comes out way earlier and since TLK will almost never last more than 1 turn you get the same yield card-wise. Plus it has support from countless deathrattle synergy cards.
This card is incredibly powerful, although I'm iffy on playing this card if the meta stays aggressive. A 4 mana 2/2 against an aggro deck might be absolutely useless, especially when you aren't guaranteed a death knight card. One big example to worry about is Potion of Madness. If a Priest steals your Arfus, you might pay heavily for that. I suppose the safest thing to do against Priest with this card is play it when you have nothing else on the board. Clearly this is only one example of how you could waste 4 mana, but I'm sure this card will see a lot of play in the late game.