If Stepan is out next game, I'd go with Fast even if Miller was available. The Rangers top 3 lines can get it done. We need that 4th line to be defensively aware and Fast has proven he is capable of that.
I'm not having Lindberg play his first NHL game in the playoffs. This isn't Kreider.
It's a toss up for me, personally. If you were to tell me that Fast was playing in that spot, I wouldn't mind in the slightest. But Lindberg, to me, is going to turn into a younger version of Dominic Moore. Whether he has that effect at this stage in his career remains to be seen, but that's the type of roll he can play, and we all know how successful that line was with Moore on it.
Fast is kind of just there, on that line. Lindberg might make it as good as it was when Moore was on it, although being that it would be his first game, he'd be extremely prone to nerves and such (most likely) and thus a higher risk.