well, the fourth day does involve a very steep descent and a very steep ascent, as well as a tolerance for crowds.It's doable if you are in very good shape, but it's tough on knees. On the plus side, if you're just feeling wiped out once you hit the Valley floor, you could instead opt for staying at the backpacker campsite in the Valley and then take the bus back to your starting point. You could even just dayhike up the Falls trail and then take the bus onward. No crime in making adjustments when your body needs it.
By the way, do you have a permit to hike up Half Dome? The rules changed a couple years ago, you have to have a separate permit for that. Also, be sure your bear cannister is not stored in your pack when you hide it in the bushes (I'm sure the rangers will give all the necessary advice on that)
Anyway, congratulations on getting an itinerary through some of the finest mountain scenery in the world!