If they loved GC, tell them to visit Arches and Canyonlands just a few minutes south of i 70 in eastern Utah. At least a quick drive through, although hiking there would be great.

The parks in Utah are beautiful, and if you could squeeze in Colorado you won't regret it.
On our cross country trip we went a southerly route on the way out, and on the way back we went through Colorado, up to Cheyenne Wyoming and across South Dakota on Rt 90 to see Mount Rushmore. On that way back he could stop in Chicago on the way back. Though Im not sure if you could really do it all in 4 days.

Heres a suggestion:
Day one - leave 5 am to LAs Vegas (arrive 10am)
depart Las Vegas 1pm - for Bryce NAtional Park. (arrive 5pm)
Checkout Bryce. Overnight near bryce.

Day two - Check out Zion in am
11am depart for Colorado (arrive Boulder 9 or 10pm)
Depending on their interests either overnight Boulder or Estes National Park.

Day Three - Check out Boulder or Rocky Mountain National Park in Estes.
Depart Colorado 11am for South Dakota.
Arrive 6pm - Mount Rushmore
Drive through Badlands National Park
Depart for Chicago 10pm - drive all night and arrive Chicago 11am.

Day Four - Check out Chicago for an hour!
Depart Chicago 12pm.
Arrive in CT 12am.

This trip would of course be more enjoyable with a few days tacked on.