In order to INSTALL any other operating system, you have to make some space for it first. There's no way around it. Alternatively, you can try some linux live distros which will run in RAM. All the settings will be lost once you reboot the computer.
If your disk is not completely full, you can defrag your winxp, then run one of those live CDs/DVDs and shrink your windows partitions to make some space for linux and then install linux on it.
btw, is there any specific reason why you want to run rhel4. It's quite old. Try CentOS 5 (5.4) (which is a free clone of RHEL5 (5.4) or Fedora for a bleeding edge distro that is based on RHEL (or shall I say RHEL is based on Fedora?)
... welcome to LQ.
's a link for fedora live cd