This would very much depend on the film, since Disney tends to mess with the animation on each release. The 4k version of Cinderella far exceeds the Blu Ray version, so that is definitely worth the upgrade.
I imagine the difference for Lion King, BatB, Aladdin, Little Mermaid are negligible though the HDR might be worth the upgrade. Blu-ray.com does reviews of each new release, and might be worth looking through for any titles you want.
The digital versions that are included should be 4k as well. One benefit of the discs is they are a much higher bitrate for the video and the audio. In theory, assuming a high quality enough system, the disc should look and sound better than the digital version.
But so much of the potential benefit of a 4k disc is the HDR or if it was properly remasted for the 4k release (Raiders, Jaws, Gladiator, Cinderella, Alien, etc. all look incredible in 4k compared to their Blu-Ray counterparts)- since so many films were mastered at 1080p then upscaled for the 4k release. And HDR relies on a properly calibrated display to actually make the image look better- and 99% of the displays out there in people's homes are not calibrated properly.