Poor? From what I've seen, it's inline with DS DQ remakes and One Piece Warrior (IIRC) which are the closest thing we can compare this spinoff (not that I am saying these are good figures mind you).
I think it is unfair to judge the PS4 japanese sales based on DQ spinoff. And if we are comparing MH4U, we should compare that to Bloodbourne which maybe around 400-500k (someone has updated figure on this?) in NA.
D5 on the other hand is kind of sad. I love this series, I hope it did good digital which I kind of doubt
"inline with DS DQ remakes and One Piece Warrior" = poor. SQEX decided to ditch DQ IP in the West because of DQ remakes sales - mind you, they were quite smaller projects with respect to DQ Heroes, and surely way less advertised.
I also feel Bloodborne doesn't necessarily fit into the "Japanese game bought by Japanese game lovers" category - it was advertised as a primary first-party IP, and is brutal and mature-themed action, which surely fits well the current PS4 audience.
Explain Bloodborne's sales.
Also, Zestiria did well considering it's now multiplatform.
And do you expect Persona 5, FFXV, KH3, Dark Souls 3 to all bomb?
I also feel Bloodborne doesn't necessarily fit into the "Japanese game bought by Japanese game lovers" category - it was advertised as a primary first-party IP, and is brutal and mature-themed action, which surely fits well the current PS4 audience (more in line with Resident Evil, for example, than Disgaea).
Zestiria sold ok - even though the first Tales of game on platforms like 360 and Wii debuted way better.
As for P5 & co. - I don't feel they will bomb. After all, big IPs should be safe, because they are able to catch a wider audience. Not expecting KH3 to replicate PS2 numbers, of course.
Putting aside the fact that DQ Heroes is not even a JRPG (comparing it with muso-like games is indeed better suited) the "lot of jRPGs" you're mentioning are most likely announced, but not out yet, right ? Things like Star Ocean V, FFXV, Kingdom Hearts III, DQXI or Persona 5 ? Because right now, I don't have the sensation PS4 is crawling under JRPG. I may have missed something, though.
People are buying a platform also considering the perspective of future games - that's how, for example, sold PS2 in the first year and a half: before the heavy hitters came, audience was already there.
It performed w.r.t to what it did historically. That is fine in my book.
Some people are using these numbers to blame the platform, which is rather stupid.
You think SQEX would have expected more since they started to publish DQ by themselves again in Western countries.