How long to spend at the Grand Canyon???? This is a much debated question.
For some it is just a big ditch and a few hours is enough. Some will spend a day or two enjoying the views from each overlook.
Others will spend days. Enjoying the views from each overlook in the morning and then again late in the afternoon. As the sun crosses the sky the views of the Canyon change with the shadows. This appeals to those that like to photograph the Canyon with the changing light.
At least one evening has to be spent at an overlook at sunset. They are amazing. The same for a sun rise.
If you plan your stay, I think you will really enjoy your 4 days there.