I don't think anyone is expert enough to know where the best place to live is, otherwise we'd all live there! No-one is going to know what is best for your son because we don't know him as you do. There are so many variables; travel distance, single-sex or co-educational, large school or smaller, school specialisms, extra-curricular activities, religious preference and so on. There are also plenty of comprehensives in Berkshire and Upper schools in Bucks that will provide a good education, some of which might do a better job than a grammar school. It all depends upon the child and where he is best suited.
I suggest narrowing down your preference to a particular town or towns in each county, visiting a few schools and reading some Ofsted reports and see what you think.
As for preparation in year 4, I believe that none is needed and that children should be left to benefit from and enjoy their schooling, with parental support of course. Worry about preparation later in year 5 if you and the school genuinely believe your son is suited to a GS.