Heat your Instant Pot, choosing the Saute-High function. Add the oil and let it heat for a minute.
Add the cabbage or kale to the pot and cook, stirring occasionally, until it's softened, about 5 minutes.
Stir in the garlic and cook for another minute.
To the pot, add the potatoes, water, cream, 2 tablespoons of butter, salt and pepper. Attach the lid of the Instant Pot and make sure that the valve is in the sealing position.
Choose Pressure Cook-Manual for 7 minutes.
Do a manual release by switching the valve to the venting position. Be careful as it will steam!
Mash the potatoes. I like to use a potato masher as it doesn't overmix the potatoes. You can use your preferred method. Taste and add the remaining 2 tablespoons of butter, if desired. Mash until your desired consistency.
Add extra salt and pepper, if desired.
Store leftover potatoes in an airtight container in the refrigerator for up to two days.