I would like to switch from a 5 day week to a 4 day, but maintain a 40 hour working week.

Would the 10 hour days affect your ability to be productive? I hate our public transit system so if I could reduce my transportation by 20% I would be happy.
If other developers who work 10 hours shifts could be clear as the their experiences with it that would help me. 

I think my boss is flexible enough that he would be cool with it.

The literature on the subject points to the harm that long days (e.g., death marches) do. 

It is 


Impossible for humans to work productively for extended periods of time1,
Unrealistic to expect people to work more than 2-6 hours in an 8-hour day2, and
Detrimental to overall quality to force people to work longer hours3


1 Nöteberg, Staffan. "Pomodoro Technique Illustrated". 2009. Pragmatic Programmers. pp 31-33
2 Brooks, Frederick. "The Mythical Man-Month". 1995. Addison-Wesley. pp 87-94.
3 DeMarco, Tom and Lister, Thomothy. "Peopleware: Productive Projects and Teams". 1999. Dorset House. Chapter 3-4
