I would always take the new version. Only if the version had been released with a few months, and it had been a major upgrade, then I would consider taking the previous version.
So to your question, then I would NEVER upgrade anyone to a version 4.0 as this version is almost at it’s end of support.
I would upgrade to Navision 2009. If your customer had a lot of modifications and upgrading all the forms and so to the new pages and forms would be too much, then you could eventually just stay on the classic client and not the role tailored client.