I did a similar thing in May last year, did the south downs way from Winchester. We did it dressed as Romans to raise cash for help for Heroes. We tried to keep it authentic and carried our kit in hessian packs we made our selves, with hessian and rope. This meant our kit had to be extremely light. We wild camped the whole route and did it over five days, approx 20 miles a day. Because of our kit it was fairly tough going, comfort wise, ropes digging into your shoulders and a bloody great roman helmet on your head.
But one thing I did learn from it when we set up camp each day regarding kit, was I found it more enjoyable, because of the fact kit was kept to an absolute minimum.
I dont know how hardened you are to this distance over four days,but from my experience dont for get some GOOD blister plasters and foot powder. As for water we were fine as there was water points and they were marked on the map. If you dont have this advantage I would fill up at every opportunity, Farms being a good bet.
You say your doing it self sufficient, does this mean your carrying four days food. We cheated on this one, The Army supplied us both with ration packs and we went out and buried them along route the week before. Luckily no wild life nicked it which was our biggest concern.