4k Laptop to QHD 1440p

@Cyber Sec.  As a general bit of advice, when asking about technical questions like this, it's often a good idea to identify the specific models of items involved in your plans, such as the display and actual system, not just quoting specs.  That might allow someone who's familiar with those items to call out any potential issues or limitations that might exist with those particular items (or using them together) and that might not be obvious.

I'm also assuming that your curved 34" QHD display would not be an actual QHD display (2560x1440) but actually ultrawide QHD (3440x1440).  But either way, I personally wouldn't recommend trying to use a 15" UHD and a 34" QHD display simultaneously.  You'll probably want to run your 34" QHD display at the default 100% scaling, and if you're running your built-in display at 250% scaling, that's going to create a huge GPU scaling gap to bridge.

Windows does allow you to set different scale factors for different displays, but it will only ever render everything at a single scale factor, and it uses the scale factor of whatever display was primary at the time you log in.  For any other scale factors that come into play on other displays, Windows will still render everything at that one scale factor and then blow up or shrink down the rendered content to fit the scale factor being used on the non-primary display where the content is being displayed.  As you might imagine, that mechanism doesn't look as nice as rendering for the scale factor actually being used on that non-primary display in the first place.  This is especially noticeable for text, which is designed as "vector graphics", which basically means it can be scaled up or down basically infinitely with no loss of sharpness as long as you do it pre-rendering -- but if you render the text to turn it into a pixel-based image and THEN scale it up or down, it won't look as good.  That's why you might see threads on this forum saying something like, "I've noticed that whenever I have my external display and built-in display in use at the same time, text doesn't look as sharp on one of them, even though if I only use one or the other at a time, they both look fine."  They're seeing the effect I just described.  Windows will deal with multiple simultaneous display scale factors, but it can't optimize for more than one simultaneously.

This is why I recommend that people only use displays that can share a single scale factor simultaneously, or at least very close scale factors.  But mixing 100% and 250% means that whenever you're using them together, one of those displays isn't going to look anywhere near as good as it would on its own.

So if you want to extend your workspace, consider buying one or two external displays that would by themselves give you the workspace size you want so that you don't have to use the built-in display simultaneously.  That's actually a far more ergonomic setup anyway if you wouldn't be keeping your laptop on a stand.  If you like the idea of a single large display rather than separate displays, there's the Dell U4919DW that's 5120x1440, i.e. two QHD displays fused together.  Or just get a pair of regular QHD displays, which will be significantly less than buying a single display of that combined size/resolution.  Those displays are great if you're doing work in an application that would singlehandedly use all of that space, in which case eliminating bezels is nice, but if you'll simply have a lot of different applications open, then having physically separate displays can be convenient for compartmentalizing, plus it gives you a bit more physical placement flexibility and again, a lower cost.