• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
Get diplomacy ideas.

Also open up your diplomacy window and you should see a little box with a "6/4" in it, hover your mouse over it and it should tell you who you've got diplo relations with; IE an alliance, royal marriage, military access, or vassal.
 
Well, the simplest way is to cancel some of your diplomatic relations. Mouse over the 6/4 and you will see a list. Chose the ones you can effort to cancel and do it. In the future, try to avoid going over the cap.

There are other ways too, of course, but they take longer. As Kombatdoctr said, diplomacy ideas can give you a +2, which would solve your problem. I am not sure, but I believe being the Curia Controller or the Emperor might change the cap as well.
 
Go to the diplomacy tab for your country and check the tooltip over the 6/4 relations indicator. It will list exactly what the six relationships are taking up your precious diplomatic slots and you can decide whether some should be cancelled. Some could be things like military access or guarantees you don't need any more. Longer term, as others mentions, some ideas can increase the limit so you can have more than four at once without any penalty.
 
To clarify, a Relation counts as one NATION, not one agreement. If you've got a royal marriage and an alliance with a country, that still counts as one relationship. But 4 royal marriages with different countries counts as 4.
 
To clarify, a Relation counts as one NATION, not one agreement. If you've got a royal marriage and an alliance with a country, that still counts as one relationship. But 4 royal marriages with different countries counts as 4.
Yeah you can have multiple agreements with the same country and it counts as one.
 
Long gone are the days where we loaded up as Austria, RM'd the entirety of Europe, and proceeded to vassalize the entire continent by hook or by crook.