Actually, you should be fine going directly to JIRA5. Just make sure all the plugins are compatible and ofcourse read all the release notes to understand the changes for each version in between (espectailly User Directory changes etc).
And yes, do it in a test environment first!
Yes, I forgot that one. Always in a test environment first.
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In many Atlassian documents indicate that you could do an upgrade from any Jira 4.* to the Jira 5. But, it is highly recomended to do first an upgrade to the version 4.4 first and then to Jira 5.
You could see this documentation, the last paragraph.
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That could be a great option. In that page I indicated you can see in the comment part, the last comment is from Giles Gaskell [Atlassian Technical Writer], and gave a few tips for your case. In fact, it was for the Jira 4.3 version, but I think it is almost the same thing.
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