In general, the surf in June - September is alot smaller than December - March, but south beaches like Waikiki get the occasional big swell in August. So you should have no trouble finding surfing lessons and some surfable waves in August, but nothing like the monsters that hit the North shore in the rainy season, that's for certain.

With regards to your second question, the islands arent so big - except the Big Island - that one cannot see all of them without changing hotels, but on the other hand, with 4 days on O'ahu, for instance, I could handle two nights in Waikiki, and two nights at Turtle Bay (North Shore), for instance.

Hope this helps,

Ciao