That's an immense amount of hard work.
First off, Jira 6 data, while similar to Jira 4 data, is different enough that there is no chance of a simple export being even remotely compatible. You're going to need to extract, manipulate and recreate the data.
The method you pick to do that is going to depend on how much data loss you are willing to accept, balanced against the amount of work you'll need to do.
The most complex method would be to export the xml from 6 and edit it to make it compatible with 4, then export all the data in 4, merge them, and rebuild the version 4 server from that. No data loss, but seriously hard work.
At the other end of the scale - export the bits you need to CSV and import them into version 4, no downgrading of data needed. But you won't get comments, history or attachments.
There's a range of options in the middle - exporting and converting to Jelly scripts, SQL inserts is probably slightly easier than xml, but you'll have to do similar levels of data migrations, export and use the CLI addon to automate poking data into 4... and so-on
Personally, I'd be re-examining the refusal to upgrade version 4, because it's going to be a lot cheaper, faster and easier than all but the most lossy methods above.
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