Very much a buyers market as these schools are finding it difficult to fill boarding places as most UK families no longer choose this option at Y4. Have a look at the "new at boarding prep thread". If you are British and your DS is half way sensible and you can pay, they will bite your arm off for him. Do not be led into boarding at 8 because they tell you it maximises chances for "top" public schools. It is no longer the case. You can easily start at 10 or 11 and achieve the same objective.
What boarding option do you want? Ludgrove and Cothill have more of a full boarding ethos - the last two full boarding boys preps in England. Others ate becoming more flexible in their offerings.
Do you want your DS to come home at weekends? Every weekend? Friday to Monday? After games on Saturday to Sunday evening? A couple of nights a week in the first instance? In practice, even at the "full" boarding schools you will have a fixed exeat every three weeks, one extra optional weekend away each side of the fixed exeat and a Sunday outing each side of the fixed exeat. This means you can see your DS every weekend if you want. But - if say you are abroad- it also means that his friends may go home/to grannies every weekend and he may be on his own or with a very small number of overseas students. This is particularly the case at Y4. And it is not great for the child left behind. If you are contemplating Saturday nights at school, ask a lot of questions about how many boys are in on a Saturday night. Then ask again. And again. And be prepared for them to lie to you. There will be few, if any, Y4s. Also check if exeats also include Mondays. That can also be abit of an issue.
Which senior schools are you looking at? Look at the destination schools. The Berks/Oxfordshire schools you are looking at all send boys to the most competitive senior schools, so they will all get the right boy in. But if your DC is not a bright all rounder they cannot perform miracles so don't expect them to.
Do you want a small school eg Sunningdale/Woodcote? Or something bigger?100 boys in total can be very nurturing at the bottom end but can get a bit limiting for a 6ft 2 13 year old who has been there for five years. It means your DS gets to be in all the teams - but if he is a potential county level player it may be a bit frustrating playing with half the team made up of what would be third team players in a bigger school. And it means you never really get to play strong teams. So unless you are also playing outside schools it can be a bit of a shock when they move on to senior school.
Do you want to go for matches during the week? How easy are they to get to? Will you drive? Use public transport? Do you have friends/family at any of them who can keep an eye on your DS if you are not around?
All of these schools have a very similar offering facilities wise. All offer academics, drama and music. All send boys to a range of "top" schools. So- I would work back from what I expected to happen at weekends, decide on size and then visit.