The Brahma Vihārās – often translated as the divine or sublime abodes, or as the four immeasurables – can be used to focus for meditative inquiry. They are:

• loving kindness – maitrī / mettā
  • talks and readings on mettā, including the Buddha’s words on loving kindness
• compassion – karuṇā
• empathetic joy – muditā
• equanimity – upekṣā / upekkhā

Here’s a short essay by Robert Aitken Roshi on Zen practice and the Brahma Vihārās – published in Original dwelling place: Zen Buddhist essays.

And here is a 10 minute introduction to the Brahma Vihārās and Zen, by Kirk, from late 2021 –