/usr/lib/x86_64-linux-gnu/libignition-math4.so is in libignition-math4-dev 4.0.0+dfsg1-4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libignition-math4.so.4
/usr/lib/x86_64-linux-gnu/libignition-math4.so.4