Should I contact the Editor again? I have submitted my mathematical paper to a journal in November 2018. After that, it has the status "Awaiting Referee Selection" during 4 months. I contacted the Editor-in-Chief for his support, luckily, he is very nice. The result is that the status of the paper has been "under review" in March 2019 and he said that:


  I hope to receive the reports of the referees by mid-April.


However, today (April 24, 2019) I still has not received the first decision. 
Should I contact the Editor-in-Chief again or not?
Thank you for your comments.
No. 

Mid-april was last week, and his response implied that he couldn't be sure to receive the reports (who could be sure of how long reviewers take?). I suggest you wait at least one week, ideally two before contacting him.