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)
HOST localhost 0000deadbeef 5053 ISV solidangle LICENSE solidangle arnold 20141225 25-dec-2013 1 share=h min_timeout=120 start=22-aug-2013 issuer=Stephen customer=00341 issued=25-nov-2013 replace _ck=e6d67ca767 sig="60Q04580K1EYXVXRHEADA 9V4B726PH7WAG08XNGU08AG18UADEADBEEF886MV7CNJK89R6G17A265G856"
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 firstname.lastname@example.org
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.