4 weeks is plenty of time to see Denver and Grand Canyon. A week to get west and a week to get back. But you can’t tarry much along the way or spend much time at other attractions. Assume you will average 50-55 mph on good interstate quality highways including time spent for rest, fuel, and food stops. Less on secondary highways. Now use google maps to find total round trip miles. And do the math to see how many hours that will be actual highway time. Figure how many hours you can comfortably travel a day. That will tell you how much sightseeing time you will have.