Two persons should not talk privately ignoring the third, for that will grieve him.  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ يَنْتَجِي اثْنَانِ دُونَ الثَّالِثِ فَإِنَّ ذَلِكَ يُحْزِنُهُ  

Abu Salih said: I asked Ibn ʿUmar: If they are four? He replied: Then it does not harm you.  

قَالَ رَسُولُ اللَّهِ ﷺ مِثْلَهُ قَالَ أَبُو صَالِحٍ فَقُلْتُ لاِبْنِ عُمَرَ فَأَرْبَعَةٌ قَالَ لاَ يَضُرُّكَ