You should be able to upgrade from 4.2 to 4.6 using the normal upgrade mechanisms, however, please read the 4.6 release notes since a problem existed in 4.2 that requires you to make a manual DB update prior to upgrading - I'm referring to the yellow box that says:
If upgrading from RHQ 4.2 (4.3 and later are not affected) you must first make a manual change to your database. Have a look at the first note on the RHQ 4.4 release notes
When you read the 4.4. release notes, it says:
If upgrading from RHQ 4.2 you must first make a manual change to your database. Apply this change only if upgrading from RHQ 4.2, not earlier versions. Execute the following SQL to update the schema version from 2.114 to 2.115:
update rhq_system_config set property_value='2.115', default_property_value='2.115' where property_key='DB_SCHEMA_VERSION';
After this update proceed with the upgrade normally.
Also, 4.7 was just released, so if you're going to upgrade you may want to move to 4.7 as opposed to 4.6.