Arnold 4.1 and later uses what we call "version-as-date" licensing (also known as “maintenance-thru-date” licensing or "date-based" licensing).
That means instead of a version number like 400, licenses now use a version number that is in a YYYYMMDD format. For example, here's a permanent license with the version number 20141225 (on the LICENSE line)
This license will work with any version of Arnold released before 20141225 (25 December 2014). That includes Arnold versions 3.x and 4.x (since 300 < 400 < 20141225).
When you get a new license, the version number in the license is either the date of purchase, or the expiry date of your support contract.
Wrong license version for arnold
With date-based licensing, if you try to run a version of Arnold that was released after your license date, you'll get a message like this:
// WARNING : [arnold] [rlm] wrong license version for arnold 20131216, found licenses for:
// WARNING : [arnold] [rlm] arnold 400 (1 license)
// WARNING : [arnold] [rlm] please contact email@example.com
Notice that Arnold now uses a YYYYMMDD version number when it checks out a license. To run "arnold 20131213", you need a license with a version >= 20131213.
Here's a short video where I go through the install of a new license, and briefly discuss the new licensing.