It's definitely possible if you can actually stay focussed during the 4 days. For my honours dissertation I wrote 15,500 words in two days and got an 'A' for it and a first overall. But then I find that I work much better when under serious pressure - for some reason I focus a lot better in this way. BUT I never found anybody like that apart from me in my entire undergraduate degree. Some people write a lot better in bursts (ie 1000 words one day then nothing, then 3,000 another day etc.), while others seem to prefer the consistant approach (ie 500 words a day every day). Are any of these your usual approach? Either way, let us know how you get on! :)