> If there are > 4 letters, 40% of all letters are bold.

Very cool :)

I would have thought there is also value in bolding the last-x-amount of letters.

I forgot the name of the "concept" but it's been shown that the beginning and ending parts of words are the most "recognizable". You usually see a sample of that on those 'silly-facebook-posts', example:

"I cnduo't bvleiee taht I culod aulaclty uesdtannrd waht I was rdnaieg"

[1] https://www.ecenglish.com/learnenglish/lessons/can-you-read