31" is really too small for 4K.
Only if you're further than 25" from it. At any distance closer than that, you'd be able to resolve individual pixels. Until pixels are so small they are invisible, pixel density isn't high enough.
Furthermore, If you sit closer than 31" to a 40" monitor at 4K (as we all would), you'd be able to see individual pixels. So no, the pixel density on a 40" 4K monitor wouldn't be at the limit of normal visual acuity.