the change in version number shouldn't matter. my system has 8,3:
/usr/lib/libtcl.so -> /usr/lib/libtcl8.3.so
and any program using it, is most likely to use just libtcl.so and not knw any difference. if it does whoever, just try creating a symlink from libtcl8.4.so to libtcl8.3.so, and if it is as compatible as it say, you should be fine.