Congratulations, OP, for achieving offers from these four schools. It is wise to work backwards from senior school to assess which is the more appropriate prep school for that particular senior school. The three senior schools that you mentioned - Eton, Harrow and Winchester - are in a way, quite different from each other:
In reverse order:
Winchester - Generally agreed to be a very academic school for boys who have (not generally agreed) not much other interest in life.
Harrow - To a certain extent, the opposite of Winchester, i.e. boys spend a lot of time doing all sorts of things except getting to be more academic.
Eton - The best of both worlds, a fine balance between the above two schools and giving them a good run for their money - academic, sports, drama, anything . . .!
And so on to prep schools:
Ludgrove: Not generally a school at the tip of tongues of potential prep school parents. If not for the royal princes who went there, most parents would not have heard of this school. I note you have favoured this school for your own good reasons. Maybe you are local to this school judging from your username and if that’s the case, I’d suggest you do not allow this to determine your choice of schools.
Cothill: A school I’d say which is more popular with parents outside London. A good feeder school to Eton and Harrow but not so much Winchester which gives you a general idea the kind of school this is.
Summer Fields: A prolific feeder to Eton and one that impresses me much by the number of King’s Scholarships their boys won (regularly) in the last 15 years. I’ve also been much impressed by the good manners and the way they carry themselves when talking to adults of boys from this school.
Papplewick: Another prolific feeder to Eton and a notable many to Harrow. Its presence in Eton’s annual KS Election Roll is notable considering only 14 boys from the UK and the rest of the world make it to the Election Roll.