Ecologist, do you sit in the office or do you actually go out in the field? I would get the P or T models over the X ones. X1 is fancy and sleek but due to size they are more fragile and CPU speeds can't go up to max due to heat issues in many cases in small laptops. If you were a lawyer or doctor that did not need much past using Excel and wanted to look good, then X1 is a good model. For general use or higher spec setups, not as much.
I would also think twice about a 4k screen, do you actually need to fit a lot of stuff on the screen but have small icons and text? In a laptop size, 1440 is really the highest I would go.