You may find it difficult to book 4 nights only at that time of year due to peak season. Wareham forest would fit you bill iv they have vacancies. We have just returned from a wonderful break there.
------------- Albino, Can"t say fairer than that !
the Shorefield, or maybe it was Shorelands chain, they are quite decent, near Bournemouth, usually had spaces as well, tents, vans, pool, bar, near nice towns, simples.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.