I don't think 13.4.1 will be the end. That would mean just one update in almost 6 months (end of March until mid September when iOS 14 is released).Maybe? There would be just about enough time to beta that one if 13.4 rolls out by the end of March. But my guess is we’ll see 13.4.1 instead. This may be the end of the line for lucky iOS 13!
The last update of any major version of iOS before the next major release ships to public is always around late July or even August. Followed by a mid-September release of the new major-release.
I am pretty sure that there will indeed be a 13.5 (released around late May) with something like 13.5.2 or so beeing the last iOS13 release in August before 14 is officially out (not beta!) in September.