Maybe Nicholas's friends aren't as wild as DS's friends, but I know there have been times when I watched the clock just waiting for a play date to end! It really depends on the individual kids, but I still thing 4 total is plenty for overnight.
I am used to kids complying with my requests, and DS had one little friend a couple of years ago who just would NOT do what I asked him to do (like get out of the pool when it was time to get out). He also climbed up on my kitchen counter so he could reach something!

The reason I mention this is because I had NO CLUE he would do that when I invited him!
DS doesn't have friends sleep over very often, but if he does I make sure I screen his choice VERY carefully!