100% accurate, no (technically I'm emulating a portion of a display that's 75% of 4k). But I'm pretty sure it answers the question adequately of "does WP look like it is capable of scaling sufficiently and properly to be usable on a 4k display running at its native resolution", unless someone wants to put forward a substantial argument to the contrary or flat-out prove me wrong by trying WP X9 on a 4k display running at native resolution and producing results utterly different to what I've found. We can compare results with screenshots.
What I'm about to say is not a developer's insight who has studied the APIs and written my own apps with those APIs, but as a techie who has used the various scaling features that Windows has made available over the years:
When Win10 has the scaling option enabled (ie. beyond 100%), it is asking programs to show their UI elements a bit bigger, that's all that's going on (ignoring the OS UI for the purpose of this discussion). If the program has a standard menu bar, it's using standard UI elements provided by libraries designed by Microsoft. So for example in WP X9 the text menu bar is using those elements as the menu bar scales correctly. However, what it's using for its toolbars (with options such as bold, italic and underline) is either using massively out-of-date libraries (e.g. win32 api libraries from the days of Win9x) or WP's own custom libraries, then chances are those libraries don't support the functions required to respond correctly to Windows's request that the program scales the content beyond 100%, therefore teeny-tiny buttons.
Also, if WP X9 fails such a basic 'at a glance' test then it basically proves that the developers haven't bothered to test it in such circumstances and therefore there are probably going to be more gremlins beneath the surface such as what happens if the user inserts an image on to the page, are the UI elements that allow the user to manipulate that image going to scale correctly, or as I've already suggested, other UI elements such as multiple-choice message boxes (e.g. "do you want to save this document before closing Y/N" may not be rendered correctly, so for example the text of the message box ought to be able to wrap to the size of the box but doesn't, or one of the buttons is half-cut-off by the end of the message because because its position and size was static while the size of the message box was dynamic.
What I'm about to say is not a developer's insight who has studied the APIs and written my own apps with those APIs, but as a techie who has used the various scaling features that Windows has made available over the years:
When Win10 has the scaling option enabled (ie. beyond 100%), it is asking programs to show their UI elements a bit bigger, that's all that's going on (ignoring the OS UI for the purpose of this discussion). If the program has a standard menu bar, it's using standard UI elements provided by libraries designed by Microsoft. So for example in WP X9 the text menu bar is using those elements as the menu bar scales correctly. However, what it's using for its toolbars (with options such as bold, italic and underline) is either using massively out-of-date libraries (e.g. win32 api libraries from the days of Win9x) or WP's own custom libraries, then chances are those libraries don't support the functions required to respond correctly to Windows's request that the program scales the content beyond 100%, therefore teeny-tiny buttons.
Also, if WP X9 fails such a basic 'at a glance' test then it basically proves that the developers haven't bothered to test it in such circumstances and therefore there are probably going to be more gremlins beneath the surface such as what happens if the user inserts an image on to the page, are the UI elements that allow the user to manipulate that image going to scale correctly, or as I've already suggested, other UI elements such as multiple-choice message boxes (e.g. "do you want to save this document before closing Y/N" may not be rendered correctly, so for example the text of the message box ought to be able to wrap to the size of the box but doesn't, or one of the buttons is half-cut-off by the end of the message because because its position and size was static while the size of the message box was dynamic.