What to do when all 3 diplomacy tokens are in use and you need a 4th?
I think it's pretty safe to say that there are only 3 Diplomacy tokens provided because the scenarios in which 4 or 5 Diplomacy tokens are required at once are so vanishingly rare.

As I discovered from previous discussions of the finer points of the rules terminology (and I went so far as to double-check with the game designer Antoine Bauza via Twitter!), 7 Wonders is not a game where you need to obsess about the minutiae.  In unusual corner cases, things probably work in a commonsense sort of way, no need to over think then.  

There is no obvious reason why a player should be deprived of a Diplomacy token because 3 have already been taken already; and certainly no available justification for taking somebody else's Diplomacy token that has already been given out!  In the unusual event that more than 3 Diplomacy tokens are required at once, I believe the sensible course of action is just to use something else as a stand-in Diplomacy token for the turn.