Actually your issue is a little more basic than that. You're
attempting to install a nagios package which was built for centos5 on
a centos 4.5 system. That's not gonna work for you. You need to use
the el4 rpmforge packages, or install the el4 rpmforge-release
package. Centos 4 and centos5 are not really that interchangeable
because of glibc differences etc.