Here's an old thread that somewhat addresses your question (4K monitor + rMBP) -- https://news.ycombinator.com/item?id=7022050

I tried this configuration out a few months ago (new rMBP, Sharp 4K, OS 10.9.1) at an Apple store and it was not satisfactory; there were color fringes around some fonts. But this may have changed.

Of course, 4K + Mac Pro is a supported configuration, and in my experience, it looks great.