[CentOS] CONNECTING VIA SSH BETWEEN CENTOS 4 AND 5

Hi,

I am trying to set up an automatic connection between CentOS 4 system 
(server) and a CentOS 5 DomU VM (client) via ssh to enable my to back up 
development files on the server to the client with a cron process.

I generate they key pair without a pass phrase on the client and copy 
the public key to the same user's .ssh directory on the server as 
authorized_keys2.

When I try to ssh to the Server from the Client, I am still asked for 
the user's password on the client.

If I do the same with CentOS 5 for both Client and Server, I can login 
without providing a password.

The versions of ssh on the two systems are:

Client (CentOS 5):   OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006

Server (CentOS 4):   OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003

Does anyone on the list know whether these versions are compatible or 
how to get them to work together without requiring a password.

It is not an option to change to CentOS5 on the server side as that 
system is serving as a development system for a client running RedHat ES 
4 and has to have the identical configuration and be binary compatible.

TIA

ChrisG