You don't know what 'replacement level' means (it means at the level of a player that can be gotten for free at any time) and you don't know what a 4th line center is. Travis Zajac is still a very good player. He just doesn't belong on an NHL power play first unit or a first line.
To defend Daniel, "replacement level" is the standard average player at the position (at least in baseball when calculating WAR). I've never heard of "replacement level player" terminology in hockey.
Daniel is 1000% right when using that term for the Devils. They are chock full of players near the average (or below). Most of the team is filled with players who are not worth additional "wins" for the team by having them in the lineup.