You are the only one who can answer the question of how much is too much. We have made both versions of your trip outline or variations several times. For us, Yellowstone is one week, Glacier is five days and that is not counting travel time to and return. I would concentrate on YS and Glacier and the routes there and return.
We prefer heading up through Bishop and then up to Jerome, Id via US 6 & 93. Then through Craters of the Moon and then US 20 to West Yellowstone. Just don't like driving on I15. We stay at West Yellowstone now..used to stay at Madison campground when younger.
I would not try to do the Black Hills, Sheridan, Buffalo on the same trip if you only have 4 weeks. If you really want to include that, I would skip Glacier and head from YS out through Cody to Buffalo, Wy. and then to the Black Hills. Custer State Park is one of the great state parks in the country.
