And before you say you can't start prospects on the 4th line, Ruzicka last year or Mangiapane in 2019 say otherwise.
To add onto this, the Flames can easily bring prospects up throughout the lineup pending the forward mix they want to go with, but they need a very specific blend of lineup to do so.
For example, calling up Phillips and sticking him with Lewis and Lucic will have absolutely awful results, I'd feel. Ruzicka had limited success with them but once he got a real look with top 9 guys it was clear he would stick, but putting a prospect with the current form of those guys would be a death sentence. What helped Mangiapane come along greatly was the fact the 4th line had Ryan and Hathaway, guys who are essentially the definition of luxury 4th liners/alright 3rd liners.
But if the Flames, for example, were to play Dube on the 4th line at centre again like the lines a few games ago, that creates a situation where the Flames have
Ruzicka - Lindholm - Toffoli
________ - Kadri - Mangiapane
Huberdeau - Backlund - _______
___________ - Dube - ________
And you can feel confident Pelletier/Phillips/Zary have a competent puck carrying centre to work with. Even in this example you could put Pelletier with Kadri and Mangiapane, Coleman with Backlund, and then have some kind of competent puck carrier to put with the 4th line to avoid Lewis/Rooney getting hemmed in their own zone as the guys expected to initiate and finish a complete breakout.
Now whether or not that will happen, who knows, but in all reality I'm not sure the path for the Flames is 4th line development, but creating a better 4th line that can put a momentary hole in the lineup as the Flames have done this season, but instead of filling that whole with Lucic it's filled with Pelletier