Apple has taken the LG 4K display off the market.
Three years ago, the 4K and 5K displays (that are still available) were introduced together with LG. The two monitors were the unofficial successors of the Thunderbolt display, so to speak.
Now Apple no longer offers the 4K model. There is a new LG model with this resolution, the LG 32UL950. Nevertheless, Apple has decided to discontinue the 4K monitor series.
Perhaps this is also a sign that the 5K model will soon disappear as well and that Apple is offering its own 6K or 8K monitor in the course of introducing the new Mac Pro.
If you need a good monitor for any budget, please have a look at our monitor recommendation page. There we have listed suitable monitors for various application scenarios and connection types for you.