Speaking as someone who hasn't ever used 3.x for more than a few minutes:
Does the 3.x *NIX app have inputs, and if so, if there is a *NIX app running on a 3.x forwarder, will the forwarded data be in a proper format for viewing on an indexer running the 4.x *NIX app?
No. There is no backward compatibility for the 3.x UNIX App.