Do you know how old in weeks?I'll have to keep an ear out for the classic egg-laying song to identify who started laying!
4 months could be 16-20 weeks.
Do you have any 'sexlink' breeds?
And if you really want to know....
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/