Hi Anthony,
Header tags are used to indicate structure in your html document, the styling you apply to them can be anything you like and think makes the page easier to read. While it is a bit unusual to have small headings and larger paragraph fonts, it is certainly not black hat. Is it worth using h3 and h4? You are probably not going to see any seo benefit from using them, but if your document structure makes sense to have them, then use them. Can you use more than one h2? Yes! Check out this post by Yoast which should answer most of your questions regarding structure and proper usage.
In regards what text is best, I would go with your second 'more readable' suggestion. Write your content with users in mind first, it is easy to get caught up in minor variations with the idea that it might make some difference to your rankings. 99/100 it wont, and you will just get a page that reads a bit oddly for real users.