This plugin incorporates the pear php_debug library into wordpress.
No, this plugin does not work with php4 because the debug library pear::php_debug is php5 only.
No, pear is not required for this plugin to work
I imagine this plugin will be most valuable to wordpress developers generally It will also be good for people who are having errors on their site. This plugin also measures script execution time so It is also good for people who want to measure the performance of their entire site or a specific part of their script.
You can see the pear demo here: Pear Demo.