after usign my 4 foot platten and knowing that nate could me ke a 72 inch i saw no use
the grind over 4 foot is so shallow that its not really worth it in most cases. i have ground some little pettys that were about 1.25 inch tall .100 thick and the 4 foot platten makes a nice thin edge one them tho
If you mean that the table (or body) of your mioroshi is concave, I think that's pretty normal for debas. Usually I clean mine up with either left over mud and/or wet/dry sandpaper on a cork.
If you mean the actual bevel -- soft steel/hard steel jigane/hagane -- is slightly concave, that's pretty normal too for cheaper end offerings. It will be gone in time with sharpening. And oncave back side is what to be expected for a deba, right? And you'll want to keep it that way.