IntelliJ IDEA 2017.1 (jetbrains.com)

On which OS is this a problem? On Windows it is working fine for me (with 4K).

You need to provide some more details here. I've been running Idea on 4k panels using FreeBSD, Macs and Linux for 15 months without issue.

I run IntelliJ IDEA (and AppCode and PyCharm and CLion and RubyMine) on an iMac 5K with a second 5K monitor attached, and on a Mac Pro with two 5K monitors attached. I've seen nothing like what you're describing.

This may not be a 4k issue. I had this issue when I first used the software on a sub-1080 monitor. I can't remember what fixed it but do make sure you have fast type mode enabled in your settings, or whatever they call it now.

I had gotten used to the QWERTY like keys.