And who do they play with? Some players, like Roy this year, are better when they have better teammates. If you stick guys on the fourth line they may never show what they can do.How much does a player touch the puck with 4th line minutes? How much do they get to experiment with their game? How much confidence do they get?
There's a lot of reasons why spending 4th line minutes in the NHL is worse than quality minutes somewhere else.
It doesn't matter as much for low skill guys, they can play fourth line and chip it off the glass. The guys with offensive potential need to play with other guys who can score. Playing any serious prospect on the fourth line for significant minutes is likely to block their learning unless you have a crazily good lower line. Better to play the future scorers on scoring lines in lower leagues.