I am trying to create a timelapse using Canon EOS 5D Mark IV using the built in video timelapse feature. I've set it up, and created a 20 minute timelapse, everything is smooth except that it's 1080p, not 4K. How can I let it record 4K timelapse as it should be well within the limits of a device that can record 4K at 30fps? (other than intervalometer + creating a video from image sequence in postprocessing, of course, which is the best option in terms of quality but terrible for my shutter cycle)
This is a limitation of the way the EOS 5D Mark IV records 4K video.
When recording 1080p video the camera uses the entire area of the sensor and uses binning to reduce the sensor's 6720x4480 pixel resolution to 1920x1080 pixels per frame.
When recording 4K video the camera only uses 4096x2160 pixels in the middle of the sensor, which crops the field of view by a factor of 1.7X.
In order to use the full area of the sensor for timelapses you are constrained to using 1080p in video mode or the full camera resolution in still imaging mode.
You can reduce the wear and tear on your shutter curtains and mirror assembly by shooting in Live View still imaging mode with one of the LV silent shooting modes. In either mode the mirror stays up (assuming you don't have the 'Quick' PDAF focusing option enabled). In Silent Shooting Mode 1 only the first curtain moves to end the exposure and then reopens. In Silent Shooting Mode 2 only the 2nd curtain moves during the entire sequence. By alternating which Silent Shooting mode you use in Live View for each timelapse session you will halve the number of curtain movements on each of your shutter curtains, and reduce mirror movements to once per session.
For more on how the various Canon silent shooting modes work with the similar EOS 5D Mark III, please see What's the difference between Canon's silent and non-silent shooting?
That is just a limitation of the built-in Timelapse Video feature. Canon documentation says you can only choose 1080p with 29.97 or 25 FPS. The speed only changes the playback speed but you have no choice about resolution.
To get a higher resolution time-lapse, you must assemble a sequence of images yourself as you have suggested.