Hi @adam-tj
The current latest branch is 33.x.x. Version 4.x.x is the previous version, which is still being maintained. This means that it still receives bug fixes, just as multiple Nextcloud versions (currently 32 and 33) are maintained in parallel.
For a detailed explanation directly from a developer on the Desktop Team, see here: Why are there new 4.0.x releases still being released? - #9 by iva.horn