Personally, I do not think there will be season 4 shugo chara. This is because manga already ended. The main story have ended. Now there is Shugo Chara Encore, which chapters mainly concentrate on couple story. The 1st chapter is about Utau and Kukai. The 2nd chapter is about Nagihiko and Rima.
So basically story have already ended. The ongoing manga now I don't think will have a lot chapter. So I don't think they will be season 4. Unless they decided to have Shugo Chara Encore as a new shugo chara season.