How to make plain(cut) shape? I'm sure the question was asked a lot, but i cannot formulate it for google.
I have this quadrangle
When look at it from side, it's not plane
While zero smooth it's two triangles with different shades, I would make it into one quadrangle with single shade. Any easy way?
This is called non-planar geometry.
Shading issues are typical for such faces.

To make it planar, just scale it along it's Normal to zero.