Not remotely. As a rough rule of thumb, a very generous ratio would be 1.5 weights of dough to 1 of butter. You could go 2 : 1 (8 : 4, in your case) but that borders on the scant.
Depends what you're trying to do. If you're going for the sort of croissant you'd get from a good European bakery, go with more butter. If you want the sort of croissant you'd get from a U.S. chain supermarket, go with less. That's not a "dis," by the way. Less butter means the final croissant will not only be somewhat lighter, but won't leave you with butter dripping down your chin when you eat it.
In my opinion, a ratio in the range around 3.25 - 3.5 : 2 -- is ideal.
If you're going to go to all the trouble of making scratch croissants (more trouble than it's worth for anyone other than a dedicated hobbyist or professional baker), don't cheap out on the butter. Less is less.
Edited by boar_d_laze - 2/23/10 at 7:45am