i like the 15" bcuz i want to carry my laptop everywhere and i had 2 laptops 15" and i love the size but without doubt i will check the 4k screen how it looks
The 15" size is fine, I was saying for that size and a 4k screen. I would not use a monitor under 24,25" with 4k resolution. 1440 may be a good compromise if you want to get some more space for windows.
XPS is a high quality premium laptop, I would not get a 4k screen on a laptop though. Maybe a 17". The dot pitch is too small for text to be comfortable to read. You should see a 4k laptop screen in person before you pick one.