For Vegas, you may want to check out RTA's sister-site: Living Las Vegas.

There are no shortage of National Parks and landmarks that could be along your way. If you follow the most direct path of I-80/I-76/I-70/I-15 you'd be going right past places like Rocky Mountain NP, Black Canyon of the Gunnison, Colorado NM, Arches, Canyonlands, Capitol Reef, Bryce Canyon, and Zion NPs. There's many more options you could also consider - including the grand canyon - if you divert more towards the 4 corners area. Keep in mind that 4 full days is the starting point, and you'd be looking to add more time to start exploring most of these places.