Bump the Capstone library version to 4

This port is dependant on both the Capstone library and the Python
bindings for Capstone. The version of the library has to be bumped to 4 as
the bindings version has to be kept in sync with the library version in
order to avoid build problems.

Approved by: portmgr (build fix blanket)
Differential Revision: https://reviews.freebsd.org/D23921