Hi, author of the blog post here! Yes thank you for catching this awful typo, it's fixed now! I did write "4000 or 5000 IU of Vitamin D" everywhere else in the article -- main text, conclusion -- just my luck that the one place I mess up is right at the very start.

(Do not take 5000 mg, that's 200,000,000 IU. You'd have to chug dozens of bottles per day)