Monitors are tricky, because of a number of reasons. Most of these are personal preference and I wouldn’t say that there’s a “best” option . We use 4K monitors, and ultra wide-screen displays that allow us to tile up multiple windows when we’re working on projects. Mike even utilizes a three-screen setup:
with high performance parts that are easier and cheaper to custom build http://brightmandesigns.com/myrig/
One note with 4K monitors is that the SketchUp icons and interface may be smaller.. as there aren’t many applications with high resolution artwork for their software. Not just SketchUp, but other programs as well.. you will notice smaller icons. This isn’t an issue though, you can adjust to a larger HD setting that will work just fine.
Not a clear answer on this, just some thoughts. I’m sure any option you decide on would work just fine.