What is the major difference between a 3PL and a 4PL?

I was intrigued by the question as I didn't know the answer.  (In fact, I didn't even understand the question ... turns out its about logistics.)

1PL - you handle your own logistics

2PL- you send stuff through the mail

3PL - somebody else sends stuff through the mail for you

4PL - somebody else does everything for you, you just tell them what to do

This is the best page I found:-

http://www.globallogisticsmedia.com/articles/view/3pl-vs-4pl-and-why-everyones-getting-all-confused-about-it