I realise that there are similar questions on this problem for Ubuntu, including a question that I asked myself, but I have the same problem with Kubuntu 15.04.
My Kubuntu system is set to a resolution of
1920 x 1080 through Display Configuration settings. On system start-up (or after logging out and logging in again), the
1920 x 1080 resolution is not applied and the login screen resolution is incorrectly set to the native screen resolution of
3840 x 2160, but changes back to the correct setting after logging in.
I was interested in why this seems to be occurring, and if there are any solutions that will apply the correct settings on system start-up and after log in. For reference, my laptop is a Lenovo Y50 UHD.