my roos are usually fertile when they start mounting pullets. I do find that older girls are better at avoiding new roos. some hens and pullets never submit.
sometimes fertilitiy will depend on the lines. wyandottes have been 9-12 months and sometimes never. my GSBR was about 11 months. I do also fine eggs less fertile when it is hot....but again that would be too dependent. some roos never slow down he he
4 months is young and he may still need to master his skills. they have to learn how.....
I like your roos too
~ I will post a pic of my new roo as soon as I can.