While PHP 4 comes with a lot of new features, functions and extensions, you may still find some functions from version 3 missing. A small number of core functions has vanished because they do not work with the new scheme of splitting parsing and execution as introduced into 4 with the Zend engine. Other functions and even complete extensions have become obsolete as newer functions and extensions serve the same task better and/or in a more general way. Some functions just simply haven't been ported yet and finally some functions or extensions may be missing due to license conflicts.
As PHP 4 now separates parsing from execution it is no longer possible to change the behavior of the parser (now embedded in the Zend engine) at runtime as parsing already happened by then. So the function short_tags() no longer exists. You can still change the parsers behavior by setting appropriate values in the php.ini file.
Another feature of PHP 3 that is not a part of PHP 4 is the bundled debugging interface. There are third-party add-ons for the Zend engine which add similar functionality.
The Adabas and Solid database extensions are no more. Long live the unified ODBC extension instead.
unset(), although still available, is implemented as a language construct rather than a function.
This does not have any consequences on the behavior of unset(), but testing for "unset" using function_exists() will return FALSE as it would with other language constructs that look like functions such as echo().
Another more practical change is that it is no longer possible to call unset() indirectly, that is $func="unset"; $func($somevar) won't work anymore.