To me these tough series game endings are about:
- Putting your great players in a position to win; and
- Denying the other team the opportunity to do the same
I think they did that in game 4 - Smart may not be the first option but he got a great look at then end of regulation, after great team defense got them there. For the final part of OT, they pretty much had it set up again, despite not playing as well. It's just that 1) Brown gave Harden way to much space and Harden, as a great player, took advantage of it; and 2) Tatum messed around too much in the face of good Sixers defense, before passing to Smart (who hit what would have been a game winner).
Not sure how much this is on Mazzulla but it's a coach's job to continually improve his team's play, and not just be like "well we're satisfied, just didn't go our way". Brown has done the same thing several times before IIRC - I remember he doubled and it cost them a gut punch game a few years ago against the Wizards.