Well, I think there should be a balance depending on the relevancy of the social media to your site(s) and your budget. IMHO, I think four is enough because at the end of the day it's your website that you are really after not your social media pages. So, if you spend too much time compiling social media accounts together you might be off from what is really important.
Personally, I use only two social media[Facebook and Twitter] at most for every website I own because I believe those are the two I can handle with the amount of activities I've do deal with everyday. Moreover, the two are part of the top social media that have the kind of users I want on my website. However, it's possible I expand to others later provided that I see possibilities in them and I have the resources to manage and maximize the social media to my advantage.
NOTE: If you spend too much time running different social media, there's a very high chance that you won't really get what you need from them because your effort is somehow separated that way. However, if you focus on one, it's the otherwise.