The reason that the soname is different in Debian and upstream is explained in /usr/share/doc/libtiff4/README.Debian:
This version of libtiff is installed with a different shared library
soname from the upstream version. This is because an accidental
change to the library's ABI was introduced somewhere between 3.5.7 and
3.6.1. There are no source-level incompatibilities between 3.5.7 and
the current version, so any application that worked with 3.5.7 should
work fine when recompiled with the libtiff4 packages.
The reason that the soname is different in Debian and upstream is explained in /usr/share/ doc/libtiff4/ README. Debian:
This version of libtiff is installed with a different shared library
soname from the upstream version. This is because an accidental
change to the library's ABI was introduced somewhere between 3.5.7 and
3.6.1. There are no source-level incompatibilities between 3.5.7 and
the current version, so any application that worked with 3.5.7 should
work fine when recompiled with the libtiff4 packages.