XPS 15-9570, bad resolution of some apps at small text sizes

A couple notes in addition to the above.  If you change the scale factor within your Windows user session, not all applications will update properly, so for best results you'll want to log off and back on to see how things look with the newly selected scale factor.  In addition, using multiple displays that require very different scale factors simultaneously is never going to be an ideal experience, because Windows internally only ever renders at a single scale factor, namely the scale factor of the display that was primary at the time the user logged on.  For any auxiliary displays that use a different scale factor, Windows takes the rendered content and then uses GPU scaling to upsize or downsize it.  As you can imagine, that result is not as good as actually rendering for the target scale factor in the first place.  So for example if you had a built-in 15" 4K display where you needed a 200% scale factor and an external 24" 1080p display where you could use 100%, and you wanted to use both displays simultaneously, you'll never have a situation where both displays can look their best.  Either Windows will render at 200% and downsize to 100%, or it will render at 100% and upsize to 200%.  Between the two, the former is better, just as downsizing a high-resolution photo looks better than trying to upsize a low-resolution photo, but the auxiliary display still won't look as good as it would if Windows were actually rendering for its scale factor.  And the larger the gap between the scale factors on the displays you're using, the more noticeable this issue is.

And second, notice that I said Windows always renders at the scale factor of the display that was primary at the time the user logged on.  That's another issue.  For example, let's say you logged on to your laptop when you only had your 15" 4K display active, where you were using 200% scaling.  Then you connect an external 24" 1080p display where you'd use 100% scaling.  Even if you switch over to using ONLY that external display rather than trying to use both displays simultaneously, Windows will continue to render at 200% internally because that was the scale factor when you logged on.  So again, for best results, you'll have to log off and back on to make Windows switch to rendering at 100% to optimize for your external display.  If you're thinking that this would be inconvenient for someone who switches between external displays and the built-in display frequently over the course of a day, such as an office worker that uses a docking station at their desk but also needs to take their laptop into meeting rooms frequently, you would be absolutely correct.  Those people have to choose between either logging off and on every time they switch display setups, or else dealing with a subpar display experience in one situation or the other (built-in panel or external display).

Again, this has been getting somewhat better over successive releases of Windows 10 and as time has gone on because more applications are getting better about adapting immediately when the scale factor changes without the user having to log off and back on.  But as I said, not ALL applications will do that properly.  And even if all of the applications you use can do that, it still doesn't solve the issue of using multiple displays that require very different scale factors simultaneously.  That continues to be an issue.

If you have v1809, look at "Settings" > "Ease of Access" > "Display" and play with the "Make text bigger" and possibly "Make everything bigger".

JohnD

Thank you, jphughan, for articulating very concisely the issue I've been experiencing on my XPS 15 9575 as well.
If I use the 1080 setting on my built in 15", 1) I lose the "real estate" of my 4k display, and 2) I could use a scale factor of 100% which would allow me to switch between an external display without needing to re-login. Would making a sacrifice like this also extend the laptop's battery life?

@rcschley3  well I don't know how concise my description was, but I'm glad you found it useful all the same.   In terms of your questions, yes if you're currently using the 4K display at 4K resolution with a scale factor less than 200%, switching to 1080p at 100% will cause you to lose some real estate.  On the other hand if you're currently using 4K resolution at 200%, that would be equivalent to 1080p (just with a lot more sharpness in apps that scale properly) since 4K is twice as many pixels along each axis compared to 1080p, i.e. 4x more pixels overall.  But you won't save any battery life switching to 1080p, because you're still using a 4K physical pixel grid and you still have a brighter display and a touchscreen digitizer layer.  I believe the actual backlight has to be MUCH brighter to produce higher apparent brightness because there's less space between the individual pixels on 4K compared to the 1080p display, but I'm not sure on that.  What I do know is that review seem to suggest that the battery life penalty of the 4K display compared to a 1080p display seems to be about 2-4 hours depending on the system in question, with all else being equal.

That battery life penalty, plus the scaling issues that you encounter when running multiple displays (or switching displays), plus the scaling issues that some apps still have simply when running any scale factor other than 100% even on a single display, plus the extra weight of the 4K display thanks to the touchscreen and edge-to-edge glass is all why I got a 1080p display on the new laptop I ordered, even though I had gone for the 3200x1800 display on my XPS 15 9530 that I ordered 6 years ago.