First, I think that number (4 million) is not representative of the actual number that would usually watch the film. As someone pointed out, I know a guy that downloaded it just for bragging that he saw it before it was out.
I admit I download movies when my local library is out of copies. I did not download this unfinished movie however. I have a list of movies I want to specifically see in the theater: X-Men, Terminator, Transformers, etc. It's a treat for me, something to look forward to when I have to go to work day in and day out. I want a memorable experience, like going to the theater, other than looking at a computer screen, something I do all day anyways.
For those trashing the film, that's your opinion I guess. I personally enjoyed the movie. There were cool parts, wtf angering parts, and hilarious parts: "Did you just call me... Blob?" I know events didn't match the comics... I only wish the film had been longer.
P.S. There are a few different post credit endings, I watched the one with Wolverine in a bar.