Surprisingly, there aren't many 4 drops with good raw stats. 5/6 is really powerful and the drawback is not only delayed but not too significant unless you're playing against something like a Miracle deck or Reno Priest. You really don't want to play this against Priest.
The Dragon Tag really isn't as significant as it first seems since most Dragon decks will be able to pull off a Twilight Drake with stats just as good or even better than this. We've seen a few cards now that summon minions that cost 4 or less from your deck so maybe decks that want to use those cards can also use this.