Hi Gingko, As its a conservation area, just check that there are no covenants or restrictions regarding hedges etc but normally, ifr you only want it about 4ft high, i think you will be ok !! All those mentioned would be totally hardy in your area. The next decision you have to make is how fast you want it to grow ?? because if you choose a relatively quick growing species which will give you your hedge more quickly, the downside of that is that to then keep it to 4ft, you need to be trimming it more often. conversly, if you choose a slower growing species, the maintenance element is reduced but, it will take longer to achieve your 4ft height ??
Regarding the shape, i think that is really a matter of personal choice. another species which was in vogue was of course bay, and personally a straight square bay hedge would be very much in keeping and if you really want to go to town (and expense) you could add standard bays within the hedge (trimmed as balls) to stand above the hedge as a real feature !! One each end and, depending on the length some within the actual hedge wouls look very stylish and very in keeping. Anyway, have a look around your area and see if anything grabs your fancy that others have done. It really depends on your initial budget and how you feel about the maintenance of it ??
hope this helps, Lannerman.