Maps will show the choices.
In a four week period you could have a relatively relaxed tour to the west coast, and back. So why plan on a marathon on the way back? That implies that you will not be stopping to see anything. Are you planning on taking only one route there and back?
There are so many options for routes from which you could choose. As already mentioned, you could take one route out, and another back. All have much to offer and you are bound to find things to fit the interests of you and your friends along the way. So why not evenly divide the trip between there and back?
Best thing for you now, sisnce you still have ample time, is to get some great maps of the many States through which you wish to travel, and have a good look at your options. These are not always clear within the confines of a computer monitor. There are so many routes from which you could choose. As well, maps clearly show which are scenic routes.
You could do Black Hills, Glacier NP, Seattle on the way out and Yellowstone, SLC Rocky Mountain NP on the return journey. That's only a suggestion, since you have so many options, and we really know nothing of your interests.
The mention above of mileage and time relates to interstate highways only. If you were to detour on one of the legs through Colorado and enjoy some of the great sights that State has to offer, you would be travelling on many scenic roads and minor highways, not to mention some of the great American Byways. These invariably take longer to travel, even without stopping to enjoy the great attractions.
If you are a member of AAA you will be able to get excellent maps there. (An alternative is a road atlas such as Rand McNally.) If you do not already have roadside assistance, it could be beneficial to take out a membership before your trip.
Lifey