slight confusion on Cisco NGIPS high availability and clustering.
Can someone please help in sorting out the confusions.
You can configure Active/Standby Failover with your 4125. The document mentioned that HA is not supported when you configured clustering but without clustering you can configure active/standby Failover.
Configuraiton guide 6.5 for active/standby failover:
But if you want to have clustering or active/active you can still do that. Firepower 4100 series—Supported for up to 6 units using inter-chassis clustering.
So to answer your question, FTD supports active/standby configuraiton and clustering also but both cannot be mixed.
just to add to my prevous response, within the clustering there is HA support in a way that both appliances interfaces are active and if one went down other device's link will remain active