Sounds like a good plan for your limit of four days. I agree that you will have trouble getting into either Slough Creek or Pebble Creek with a late arrival. Another possibility for your consideration would be to drive out the Northeast entrance through Cooke City to the
Fox Creek Campground. There are several other Forest Service campgrounds closer to Cooke City, but they don't allow tents due to bears. A few miles past Fox Creek is the
Crazy Creek Campground, which we have used several times in the past. If you go there, go across the road to the short trail up along the cascading Crazy Creek. Across the valley on the opposite side we have seen Bighorn Sheep on the mountainsides (binoculars required).
On the way up to the northeast entrance stop at the pullout on the left for Baronnette Peak and use binoculars to look for the Mountain Goats.
From the northeast entrance you have the choice of driving over the Beartooth Highway (route 212) to Red lodge or the Chief Joseph Highway (route 296) to Cody, both beautiful drives.