I don't really think you should be using a 4th line with tougher defensive minutes anyway. BART was mostly used against middle-6 lines, which seems to be about on par with the Keller and Slafovsky usage that the 4th line is getting now.
I think
@Peat 's point was more.. drawing those guys even isn't really impressive. Let's see what they do against Edmonton, LA, and Calgary who have actual depth.
Even Tampa without Cirelli is a big problem.
It is part what Gurgs says. Some teams have middle sixes that aren't that scary. Some have middle sixes you wouldn't want to meet down a dark alley. So far it's been the former.
But also BART (and variations) did see a lot of time against opposition best. Blueger played over 10 minutes 5v5 regular seasons vs all of Mathew Barzal, Auston Matthews, Tage Thompson, Kirill Kaprizov, Nick Suzuki, Johnny Gaudreau, Artemi Panarin, Alex Ovechkin, J.T. Miller, Brady Tkachuk, Nathan MacKinnon i.e. opposition best forward. Realistically that is coming thanks to injuries if nothing else, and if they can't cope with that it's a problem. Blueger edged those match-ups on xGF% and lost them 6-7 on the scoresheet, which is fantastic. It's 3-5 if you take out Thompson and Suzuki as should be middle six, which is still not an awful outcome considering the level.
All of the team needs to face tougher tests to be sure what we've got, but I think that fourth line in particular. They hid them from Suzuki. At the very least they should be able to take their turn in the rotation.
Also lets not forget the part about slipstreaming. Hockey is a lot easier when the line before has a good shift. If L4 looks good the nights the top 6 looks good, but wilts when they don't, that's an issue. L4 needs to be able to provide the energy on some of those nights.