Mantra and Bard

[Bardo]

But where in Turn 4 is it at all helpful for a Bard to have mantra?

I guess you could run up and throw it on the main tank after the 1st Dreadnaught eats spiders?
And like you said, you could use it if you hit the enrage because it's possible to heal through it for a while.

Actually though, Turn4 is one of the very few fights where you could make the case that Featherfoot is actually better than Mantra.
You can pop Featherfoot and "tank" the Phase 5 Rook that drops.
So if he's telling you to get it specifically for Turn4, then I don't know what to say... that's just dumb.

That doesn't preclude it from being useful on other fights though, so my advice to the OP is still "you should probably pick it up eventually."