This release replaces 4.2.3 which was pulled due to a syntax problem in the upgrade script.
If you've downloaded 4.2.3 but not tried to run the upgrade yet, you can trash that tarball and download 4.2.4.
If you tried to upgrade using 4.2.3 and got the fatal error, you can download 4.2.4 and safely run the upgrade again.
if you used the 4.2.3 release to install a NEW CiviCRM site, no additional action is needed. (The bug does not affect new installs.)