Seeing this post, I now think that this is a bug.
A friend of mine is an experienced Ubuntu user and had me check out this exact page on my Raspberry Pi Ubuntu Mate KiCad installation. The screen clearly shows “kicad-5” as what was attempted to be installed. And, it is not that the version information screen is incorrectly showing a version 4 string of some sorts as the software itself is missing many actual V5 features.
Using the same PPA, my Raspberry Pi runs on Ubuntu Mate Xenial, and requires the KiCad to also be of the Xenial variety (it seems) and it reports as version 4.0.2+dsfg1-stable. Since I have no way of knowing what “+dsfg1-stable” means I had no idea if this is a bug, or simply lack of development for the specific Xenial build (which I understand is only weeks old).
My friend is running an Intel processor with an Ubuntu Mate LTS OS and is running a full KiCad V5; but I do not know what PPA or variety he was using. I do know that he was using a PPA that was providing nightlies builds; but only upgraded to keep the version close to in-sync with the version that I was using.
TLTR; To me, it seems that a KiCad V5 is available but it is not obvious where to get it at the moment.