For GOS 4.3 a new patch level is available:
https://www.greenbone.net/en/roadmap-lifecycle/
This release unlocks the migration from GOS 4.3 to GOS 5.0 for GSM MIDRANGE (400, 600, 650), several GSM HIGHRANGE (6500, 5400) and GSM SENSOR (25V, 35) models.
For further information on GOS 5 please see the announcement at:
https://community.greenbone.net/t/greenbone-os-5-0-enters-new-status/
GOS 4.3.15 also includes several fixes for GOS 4.3.
In total the patch level itself covers 8 improvements.