How close do you plan to get it? How large will it be on the screen?
Is this going to be used for photo realistic rendering, or in a game?
I believe the answers to those questions will greatly influence your answers, but based on the question, it sounds like you already have your answer, which is "more than 4k" for your purposes.
It's also possible that it's not just the overall density, but the quality of the UV unwrapping. Is there much "stretching" happening in key areas?
