You understood correctly. It is a one time purchase for Fences 4.
You are entitled to support as you need it. Just post on your problem in the Fences support forum. You'll be entitled to updates of Fences 4.xx until a major version change, i.e. Fences 5.
To keep up to date, and prevent loss, remember to download the installers to updates as they are published because there is no library of old software installers.
