"The exit results if I'm brutally honest, don't really reflect the feedback"
In these circumstances you need to understand your child - academically, how advanced socially and emotionally they are and their extra- curriculum advantages - sport, music, drama, etc. Then go to the open days with your son and request private tours without son if possible (ask more targeted questions without distraction). This enables you to get some idea of best fit.
Our independent school - Head schedules a meeting to discuss suggested schools based on reports, etc. We were notified of school fairs (preferred open days, personal tours) Head's reference held a lot of weight with new school and was mentioned in our offer letter. However, if leavers destinations are not as expected, consider deciding independently based on your own research and knowledge of your child, which schools to enter 11plus exams. In addition, the extent (if any) you need to supplement learning & other skills - tutors, specialist coaches / teachers for potential sports / music scholars.
Decide whether you want co-ed or single sex and whether you want son to travel or keep relatively local - basically narrow down your selection and the view schools keeping in mind the person your son is and what would be a happy fit.
I have also known some YR4 parents who realise the school wasn't heading in the right direction to achieve the outcome school they wanted to to get an occasional place at what they considered a more focused prep for the last two years or make the move to an all through school (avoiding the 11plus, but still usually some assessment entry) How disruptive this is depends on your child.