only if you make a case that six to eight hours of deep work is somehow not possible. Perhaps you make a good faith effort to get 6 to 8 but you can't seem to do it, or you're too exhausted after 3 or 4, but you can't simply demand that you should get paid for 8 but you're only going to work for 3 or 4 because that's enough.
6 to 8 hours of deep work is certainly possible but is it sustainable? I don't think so, minds need some slack time, enter a different mode of thinking, etc. For this reason work is chunked into workable amounts which it makes it sustainable as well.
Another situation: we were estimating tasks and I challenged the estimate for a task he was supposed to do.
His reply: "ok, if I'm going to reduce my estimate then you reduce one of yours"
Normally I wouldn't care about such things, but my work depended on his and it was hard to make him follow through on his promises.
Three to four hours of deep work could be enough to complete the daily workload. Maybe he was downplaying the other rest of time that didn't feel like he worked at all even though he was in meetings and doing other BS the company required him to do.