Are there any issues with installing the latest version (4.0.3.0 (GOG-12) per download window or setup_dreamfall_chapters_2.9.0.13.exe per the downloaded filename) "on top of" my current installed version (2.8.0.11 per filename, don't recall what GOG download page labelled it as)? That is, can I install the Book 4 version over top of the Book 3 version without issue?

Perhaps uninstalling DC first but keeping the saves, then installing the Book 4 version.

That should work; it worked for me.

I know of absolutely no reported issues when installing 'on top' of the old game. Then again, deinstalling just costs you a minute. No need to take the risk.