[CentOS] trying to upgrade from Centos 4.0 to current

I have downloaded the current iso files and burned the cd's for Centos. I am
trying to do an inplace upgrade to my Centos 4.0 server.  I have tried
booting from the cd and it wants to wipe out my original partitions. I don't
see an upgrade current version option. Where am I missing it?

Thanks in advance for your help.

Dave