I am curious what is the longest an interstate goes at the bare minimum 4-lanes. Obviously that means it spends a long time away from major population centers that would cause the interstate to gain a lane in each direction. I have a feeling I am going to get a lot of American Southwest answers.
I'd expect most of the contenders to be in the Great Plains and the intermountain West, including the northern parts. But there could be methodological issues, such as how to treat lanes gained at one interchange that are dropped at the next (e.g., the third lane in each direction on I-80 between I-180 and Lincolnway in Cheyenne, Wyoming).
In Colorado and Kansas, I-70 remains at the base four through lanes all the way from Chambers Road (Exit 283) in Denver to I-470 (Exit 355) in Topeka, a distance of 521 miles.
In Utah, Wyoming, and Nebraska, I-80 remains at the base four through lanes all the way from SR 224 in Kimball Junction, Utah (Exit 145) to milepost 394.6 in Nebraska (just west of US 77 on the outskirts of Lincoln). This is a total distance of 849 miles. However, if the added lane in Cheyenne is counted as a departure from the base four lanes, then we are left with two sections of 409 miles and 439 miles respectively.
I haven't looked at I-90 in detail, but I'd expect it to have long lengths too, running from roughly Coeur d'Alene to Rapid City, and from Rapid City to Sioux Falls, and then Sioux Falls to Austin, and maybe Austin to Onalaska. After that it gets chopped up more.