It's in the "testing" repository:
http://dev.centos.org/centos/4/testing/i386/RPMS/procinfo-18-16.c4.i386.rpm
According to Manging Software with Yum manual, test repo is: " *[testing]* - Packages that are being tested proir to release, you should not use this repository except for a specific reason. It is disabled by default. "
Is curious to consider a package that contains commands like lsof or procinfo as "testing". IMHO, of course.