Heading 4 ==== is hardly distinguishable from simple text (only 105% size), heading 5 ===== is italic but too small. Adding to the docs css extension to allow page local css could be great (although runs some risk of loosing docs consistency in some pages which could be addressed as "css spam" ).
I am inclined to think that if you have that many levels of heading on one page, you might be better of splitting it into several smaller pages.
The headings provide a convenient way to see a list of items for reference in the TOC. See illustrations below. The page itself is not necessarily content heavy. Still the content it does contain may be easier to read with clearer headings.
The option of splitting to smaller pages is always there and I'm using it in other places of the Dataform docs. It comes with the price of more difficult maintenance, which is why I have suggested adding the Labeled Section Transclusion extension (https://moodle.org/mod/forum/discuss.php?d=256835#p1113710). Currently I have to create many content templates to be able to include the same content in several pages in order to reduce the number of clicks and avoid duplicating.
Btw, in the illustrations below some of the items in the TOC are included in the page via a template so it's already split to smaller pages.
Thanks Itamar, as I mentioned in MDLSITE-2329, this should be addressed in the new wiki theme (MDLSITE-2753) coming in the hopefully-not-to-distant future!