As others have pointed out, and I'm glad to see you took their advice, this trip is going to take far more than four days. At roughly 2,800 miles (by any one of three or four routes) you're looking at five to six days of driving under ideal conditions. However, you cannot count on ideal conditions in February, especially crossing the Rockies. thus the range of answers you've gotten anywhere from five to seven days.
Those recommendations are made so that you can have a safe drive. As Lifey pointed out, having two drivers does nothing to shorten the drive time. Indeed it makes it longer, since each stop will take the amount of time required by the slower of the two of you. More importantly, the non-driving partner is not going to be able to get any real rest while the car is in motion, and fatigue is going to be your worst enemy on multi-day drive completely across the continent. Plan on no more than eight hours or so of driving each day split between the two drivers.
One thing you can do to both increase your alertness while driving, as well as make the trip just a bit more enjoyable is to take two or three short R&R breaks each day. The list of potential stops just linked to cover all the major Interstates so that no matter which of the possible routes you choose, you'll have at least some notion of where you can get off the superslabs, get out of the car, take your eyes off the road, take a short walk and stretch your legs, and get some fresh air, all of which will help keep you a better more focused driver.
Finally, as noted, there are several possible routes - all within less than a hundred miles of the same distance. You should map several of them out, including where 550 miles would get you at the end of each day, but I would hold off making a final decision on which route to use until just a day or two before departure, then make that decision based on the predicted weather for each route. Unfortunately, even when you leave New York, you won't be able to know what the weather will be west of the Great Plains. It'll just too far in the future for there to be any meaningful forecast. So you should plan on re-evaluating your route for the western half of this trip once you've crossed the Mississippi. Thus I'd aim for St. Louis as a midpoint since you could go south from there, I-44(toll in parts)/I-40/I-15, or more directly via I-70 through Denver to I-15.
In any event, you've made the first best decision already, to allow yourselves enough time to make the trip safely. Now you can start thinking about what would make it more enjoyable for you as well.
Thank you this gave me a lot of clarity !