The 4th line will be fine once we're healthy, and honestly Dano is decent enough pick-up. A legit 2nd line winger isn't going to be the reason we have a successful season. It's definitely a glaring weakness at the moment, but (at least the start of this season), Kerfoot has been playing like a 2nd line winger.
The development of Jost, Kerfoot, Andrighetto, Kamenev, and Compher is what will make or break the season. We basically need another 1st line talent to carry our 2nd line. A legitimate 2nd line winger is a player like Niederrieter, Krieder, Smith, etc. Those players are great and would definitely boost our 2nd line, but they won't help turn it into a legit 2nd line unless Jost/Kerfoot/etc. develop as well.