Just to throw a spanner into the works.. are you sure its agricultural, four acres could be a pony field - which is a different classification to agricultural!
Actually just grazing a horse/pony on a field
is agricultural. Grazing animals is classed as an agricultural activity, be it cattle, sheep, alpacas, or even something you might regard as not agricultural in the least. So long as it grazes then it is agricultural.
However,
charging for keeping equines, ie having a livery, is a business and as such is not classed as agricultural and requires planning permission.
There are actually, from a planning point of view, 6 different classes of equine keeping, but very few people know of them all.
I know this isn't exactly what was being asked, but thought I'd mention it in case you were thinking of equines among your other options.