I’ve just marked the HD image/video player as compatible with 4K images (see this commit). As a prerequisite the asset upload call now accepts images with a resolution of up to 4096x4096 (up from 2048x2048). Combined this should allow you to play 4K images on a Pi4.
As before, the info-beamer backend has a few checks in place that prevent your from accidentally assigning content to devices that cannot play them. This also includes 4K images. Older Pis cannot decode them yet (see below), so right now a Pi4 is required. The asset selector will prevent your from assigning asset unless all assigned devices are capable of playing them:
Similarly you cannot assign a setup that includes 4K images (or HEVC videos) to older Pi models. If you want to try out this new feature on your Pi4, either import the package or update your existing package by clicking the Check for updates button on the package page. Feedback is welcome!
In the future an upgrade to the core info-beamer pi software for Pis up to and including the Pi3 might be released to allow 4K image playback on them as well. Although to be honest that would be rather pointless as they are no capable of generating a 4K HDMI signal. Any additional image detail is lost as a result. So adding support isn’t a priority at the moment.