Greetings Pierre,
You can upgrade directly from Hauptwerk 4 (4.2) to Hauptwerk 6. Looking in our database, I see that you have Hauptwerk 4 BASIC edition. The new Hauptwerk 6 LITE edition (replacing the Basic Edition) is only available as a subscription ($12.99USD/month), or you can subscribe to Hauptwerk 6 Advanced - subscription at $29.99USD/month.
Since you prefer a perpetual license, you can buy the Hauptwerk 4 Basic to Hauptwerk 6 Advanced license ( https://www.hauptwerk.com/upgrade-basic-v4-to-advanced-v6-perpetual/) for $389USD.
You can learn more about the two editions by visiting our website (https://hauptwerk.com/editions)