3-4 solid hours of productivity is my sweet spot as well. Though it took me a while to figure it out.
There are times when I have pushed 6-8 productive hours per day for 2-3 weeks but what usually follows is 2-3 weeks of "unproductivity". Over the long run this is unsustainable and has caused me to switch jobs in the past.
can do more but the 4 are the most productive hours. an extra 2h are half productive (which i do anyway due to work req), an extra 4h even less so