これ、意外に泣けるかも。

20041001:
The following libraries had their version number bumped up:
/lib/libm.so.2 -> libm.so.3
/lib/libreadline.so.4 -> libreadline.so.5
/usr/lib/libhistory.so.4 -> libhistory.so.5
/usr/lib/libopie.so.2 -> libopie.so.3
/usr/lib/libpcap.so.2 -> libpcap.so.3
FreeBSD 4.10 versions of these libraries will be added to the
compat4x collection. If you expect to be able to run old 4.X
executables you will need to remove the old versions of these
libraries. However note that any 5.X executables you have built
will stop working once you remove those old libraries. You should
have all your ports/packages rebuilt before removing the old
libraries.