Can 4.2.1 Forwarder send data to 4.1.6 Indexer?
From the online documentation:
"The universal forwarder is backwards compatible with older Splunk indexers. You can forward data to any Splunk indexers, version 3.4.14 or above. "
4.2.1 Forwarder is backwards compatible with 4.1.6 (I ran mine like that for a month or so before upgrading).
I'm not sure about this arrangement, but in our environment we had 4.2 universal forwarders sending to 4.1.4 indexers.