depends what you want to use it for. I print stuff on 2meters wide; even at 300 dpi 4000 is lacking. Movie industry standard is 5.5k and for mattes it's 9k. For concept work then 3k is enough; but for non-screen presentation you really need to go beyond 4.
Well, in the end it's preference. Feng Zhu says he likes to work at 6k and up for productions, people I worked for said the same. But there is certainly no rule against going over 4k, just takes more time.