If I had a huge house....
And my kids would have loved it.
1) You would know your niece; i don't think most 4-year-olds would be too intimidated by a super-large plushie, but some kids are sensitive - to make sure, you may want to run this idea by her parents.
2) A more important reason to run it by her parents: THEY are the ones who have to deal with the amount of space it takes up in their dwelling place. My kids really loved the super-sized (non-Anime) plushies and inflatable Pikachu chairs their uncle and aunt got them several years ago, but for ME, it was a huge pain having to maneuver around them and so forth. We don't have a very large house... The chairs are now in deflated storage mode (can't get rid of Pikachu!), and the stuffies ended up at Goodwill after about 5 years of enjoyment (the kids grew out of them, and there's not a lot of play you can do with something that big.)
Now my kids are collecting kawaii smaller Anime stuffies / Totoros (which we have more room for, yay!), which incidentally are easier to cuddle in bed, and will provide many more years of play (they dress them, take them places, etc.) than a large conversation piece. And it doesn't leave me cursing out their toys...
Just food for thought. (As a grownup, if I had sufficient space, I'd LOVE a giant Totoro!