Diagnosis
This is a lens alignment or curvature of field issue. The lens assembly may have shifted, or the projector is suffering from significant optical aberration, often worse in the 'Bright' or 'Dynamic' lamp modes due to optical compromises.
How to Fix It
First, use manual focus and zoom. Display a grid test pattern. Adjust the focus ring slowly to find the best average sharpness. Use the lens shift dials (vertical and horizontal) to re-center the optical path. Ensure the projector is perfectly perpendicular to the screen. Keystone correction degrades corner focus—set 'Keystone' to 0.
If That Didn't Work
Switch the 'Color Mode' from 'Bright' to 'Standard' or 'Movie'. The 'Bright' mode uses an offset in the light path that can exacerbate corner softness. If the corners are still blurry, this is a lens limitation or defect. The PX701-4K uses a fixed, non-interchangeable lens.
Heads Up
Maximum corner sharpness is achieved only when the lens is at its neutral zoom position (neither fully wide nor fully telephoto). Adjust your projector's physical distance to the screen instead of using extreme digital zoom.