Generally, you can, because the format of the exported xml is not that different between versions. Go to the list of workflows in 4.2 and look for the xml export option. Use it, and take the file it creates, and import it in 6.x
See https://confluence.atlassian.com/display/JIRA/Sharing+your+workflow for some of this, but bear in mind that JIRA 4 does NOT have the "export to marketplace" function, it's just xml export.
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