I can understand why you want to know this (it's a 33% distance variance when trying to plan a multi-week trip).
Ultimately it's going to be up to whatever the weakest link (that sounds negative, but that's not the intention) in your traveling party dictates. I can happily drive down a full tank of gas (400ish miles) without stopping (then refuel and repeat). On the other hand, the wife needs to stop and stretch her legs and get some fresh air every couple hours (and the kids don't mind either) so that's what we do.
As long as we maintain that cadence we've knocked out several 1000+ mile "days" (often starting when the kids sack out in the evening) on different trips. However, we're all on the same page about wanting to have those extra days in Yellowstone / Glacier / Yosemite / Disney / the Keys / etc. if someone in our party couldn't or didn't want to drive more than 300 miles per day we'd have to adjust our plans because what's the point in taking a trip someone isn't going to enjoy...
So... If you're asking me, sure 400 miles is no problem, it would be a short "long" driving day in our plans. But... You shouldn't be asking me, you need to see if this is an acceptable distance (time) for your partner, then work from there.