macOS applies something called "font smoothing" (image) which is adding a layer of pixels to your text. The effect is that text is made a little more bold—not "bad" but it isn't exactly as the typeface designers intended.

Here's a good example:

The thing about 4K on macOS is that its sharpest setting will be when you make it "look like 1080p" which means its sized like a 1080p monitor but its 2x sharper. If you make it "look like 1440p" which is what you're used to, macOS will draw a 5K image and scale it down to "look like 1440p" but it doesn't fit the pixels on your display 1:1, so there is a slight blur or its not as sharp as native "looks like 1080p"—but it will still be sharper than your 32-inch 1440p display, so I still recommend you move forward if sharp viewing is a priority. The only sharper alternative is to buy Apple's 27-inch 5K display at $1600 (although $1300 refurb) or a 32-inch 6K display (Dell and Apple make them).