I also have this issue. It does seem like a mermaid issue, and not a new one 
There’s a temporary workaround, which is to override the scaling on obsidian. For those who have never done this:
- Right-click obsidian.exe
- Select Properties, then the Compatibility tab
- Click, “Change high DPI settings,” (near the bottom)
- Turn on “Override high DPI scaling behavior” and set it to “System”
Hit ok, and apply. Then relaunch Obsidian.
I say temporary, because it softens the appearance of the software which defeats the point of a 4k monitor, but then again, so do gigantic mermaid graphs.
As a bonus, it also address the issue of the the mouse overlapping the hovered node name in graph view on 4k monitors. I hope that helps!