What four? It’s when a mendicant meditates by observing an aspect of the body—keen, aware, and mindful, rid of covetousness and displeasure for the world. They meditate observing an aspect of feelings … mind … principles—keen, aware, and mindful, rid of covetousness and displeasure for the world. Rightly speaking, you’d call these four kinds of mindfulness meditation a ‘heap of the skillful’. For these four kinds of mindfulness meditation are entirely a heap of the skillful.”

Linked Discourses 47.46 5. Freedom From Death Restraint in the Monastic Code

Then a mendicant went up to the Buddha, bowed, sat down to one side, and said to him:

“Sir, may the Buddha please teach me Dhamma in brief. When I’ve heard it, I’ll live alone, withdrawn, diligent, keen, and resolute.” “Well then, mendicant, you should purify the starting point of skillful qualities. What is the starting point of skillful qualities? Live restrained in the monastic code, conducting yourself well and resorting for alms in suitable places. Seeing danger in the slightest fault, keep the rules you’ve undertaken. When you’ve done this, you should develop the four kinds of mindfulness meditation, depending on and grounded on ethics.