Hi Bru,
Sounds like you've done a lot of research already. Camp 4 is popular, but it's a good place to meet other campers. After Labor Day it should be less crowded in Yosemite as school has started and the weather is cooling. Early Monday or Tuesday or Wednesday are good days to get a spot at Camp 4. You want to find out who is leaving and grab their spot.
I've backpacked several times near North Dome, but never overnight on it, but others have. It could get windy, but there's a great "in your face" view of Half Dome and it's wonderful seeing sunrise too. The easiest way is to backpack in from Porcupine Flat. Park your car there and backpack in and out from there. I usually camp where the North Rim Trail crosses Lehamite Creek (a mile or so west of North Dome).
About 1/2 mile north of North Dome is Natural Arch--it's a natural arch weathered away from the granite. Do not miss it. From the trail just follow the ridge up north from North Dome to the arch. The arch is shown on some maps but not all.
You will need a Wilderness Permit. Half are available by reservation and half are available first-come, first-served--I'm not sure you need to reserve in September as the crowds have gone. You can reserve online at
https://www.nps.gov/yose/wilderness/You also need a Bear Canister, but you can rent one from where you get your wilderness permit. There are bears around North Dome. I saw a mother playing with her cubs once in a meadow near North Dome.
Don't miss the other parts of the park though. I recommend seeing the Wanona Grove, the grove of Giant Sequoia trees at the southern end of the park. Hike up from Yosemite Valley from Happy Isles and see Nevada and Vernal Falls.
Drive to Tuolumne Meadows and hike around there. You can hike down a trail along the Tuolumne River and see falls at Glen Aulin and Waterwheel Falls.