It's in 'Section: libs' in the control file in the .deb.
I downloaded it from the link of a few posts above so that I could unpack it with dpkg-deb and take a look at the control file. In any case, if you're accessing it from a repository then you can also do 'apt-cache show libffi4' to get an idea of what it requires etc. EDIT: You can also do 'apt-cache policy libffi4' to display which repository you're getting it from. This could be useful in case there are several variants around.