According to more recent posts in that thread that's not the case. It applies to both business and personal credit cards. However, some people may have a limit other than 4, plus many people don't understand which of their cards are credit cards and which are cards (or that this rule only applies to credit cards), which makes analysis of datapoints confusing. But if you search that thread for "business" and go the last page of results, you'll get for example a post from someone got denied for a 5th credit card exactly because they erroneously though that business cards didn't count.