Any way to break down that “Other” and see what it contains?
No, not without lobbying Valve.
there might be some more Ubuntu versions hiding in there.
Certainly because non-LTS versions are not in the main table as well but neither are Debian, Fedora, openSUSE, NixOS, etc.
Forget web browsing with 4GB RAM. You can completely disregard the comments recommending a “lite DE” when merely opening a modern web site will put the whole PC into crawling. The 150 MB more or less for different desktops are completely irrelevant then.
The best “newbie friendly” distribution is just plain Fedora Workstation but with only 4GB RAM it will be a pain to use no matter what.
Edit: If you’re a KDE user yourself, you’re best equipped to answer KDE-related questions.