SIP is a tool that makes it very easy to create Python bindings for
C++ libraries. It was originally developed to create PyQt, the Python
bindings for the Qt toolkit, but can be used to create bindings for any
C++ library.

Sip 5.* is in build-time usage totally incompatible with sip 4.*. It is
the last version that supports API/ABI version 12.7, which is the
version used by sip 4.* in pkgsrc, and by PyQt5.  So bindings generated
by sip 4.* and sip 5.* can cooperate, but not 4.* with 6.*.