The simple answer to you question is No! The RDBMS for 4.2 will not sync with 5.2. Since the databases will not sync it kind of defeats the purpose of redundant servers. If you are using ODBC you could setup some type of script to create a CVS file on the master and import it to the slave, but there is nothing to prevent the slave from being corrupted if an admin should happen to connect with the slave.
I have never used the appliance based ACS, so I can't answer that part of the question; however, you can probably locate a copy of 4.2 through some Cisco Vendor someplace, but it is end of life so getting it from Cisco probably is not an option.