I have used Photoshop for at least 20 years simply to convert a PNG with Alpha transparency to TIFF with the same transparency.

And to convert an image to grayscale.

Two things I have given up hope of ever seeing in Xara in my lifetime.

Photoshop CSS 5 however does not support 4K resolution. So my relationship with Photoshop has come to an end.

Any recommendations for a simple image editing app that can do these things?

Simple is better.