In a way there is only one Brahma Vihara and it’s the Metta Bhavana, but when you, with your metta, happen  to come into contact with pain and suffering, the metta automatically, or rather spontaneously, becomes transformed into Karuna.  When it comes into contact with the joy of others, it automatically becomes transformed into sympathetic joy. And similarly when your metta is extended equally towards all there is equanimity. So YOU look after metta and the other three will look after themselves.  You don’t have to think about them. They depend on circumstances.