As good or bad as you want it to be. Arch doesn't have any default gui or desktop environment, so it really depends on what window manager / desktop environment you run. Gnome3 and kde5 are both pretty good out of the box with hidpi, xfce is pretty simple to get working. Not sure past that as my experience with Hidpi support past testing was turning it off for the gtk and qt programs I use as I find 4k at 28" fine at native res/scale.

But in general, arch keeps packages as vanilla as possible, so there's no default/recommended configurations like there are with most distros.

I’m using Gnome and it works surprisingly well. All browsers and Gnome apps look great out of the box (apart from blurry icons occasionally). Qt or WxWidgets-based apps have clipped text and small checkboxes etc; they are usable but it is not pleasant. I use only two of those apps, and I rarely use anything more than a terminal, a browser and a music player, so 90% of the time everything looks great.

I didn’t have to configure anything; Gnome automatically set the scaling factor to 2 and everything worked out of the box, browsers too.

I use arch in a VM on my HiDPI XPS13 with and without a desktop environment. Support is simple, if it doesn't recognize the HiDPI automatically (I could understand from inside a VM but I would suspect it running on hardware), support is just one setting in the X11 config file. With distros that don't have HiDPI support, I set the internal VM resolution to half my screen size and then set the VM scale to 2.0 within VMware. Overall a good experience.