Akash,
I would definitely always consider upgrading to the latest version, so currently 4.6.x instead of 4.4, as we keep improving the product.
Re. the license, it depends. I added an entry to the FAQ here to clarify:
https://github.com/orbeon/orbeon-forms/wiki/FAQ-:-Software-licences#you-sent-a-license-for-orbeon-forms-44-can-i-use-orbeon-forms-46-with-that-licenseThe advantages over 3.9 PE are many. I have added some answers here:
https://github.com/orbeon/orbeon-forms/wiki/Upgrading-from-older-versions#what-are-the-benefits-of-upgrading-from-3x-to-4xI recommend you go through the release notes listed here for all the versions since 3.9, and then you can get an idea:
https://github.com/orbeon/orbeon-forms/wiki/Orbeon-Forms-release-historyOn the difficulty of upgrading, I recommend you check this page:
https://github.com/orbeon/orbeon-forms/wiki/Upgrading-from-older-versionsIt's doable, and we try to be compatible, but you should definitely look at the upgrade as a small project.
We don't have scalability numbers at this time, but we keep working on performance. We have made some good improvements in the last few 4.x versions, and we have more coming up in 4.7.
I hope this helps,
-Erik