Hi Claus! Welcome to the forum. Thanks for including your location, that is helpful for anyone giving you advice.
Here is a thread that should be helpful to you. Growing Pawpaws ( Asimina triloba )
In their first few years of life, pawpaws typically do better with at least partial shade. I have my babies either under a willow tree to nurse them along (I plan to significantly trim said willow later on once the pawpaws are established) or I have a shade cloth over top of them which I move higher as needed.
If your trees are already 4 ft tall, you might only need to shade them the first year as long as they become established. I’d suggest fairly regular watering and a bit of high nitrogen fertilizer starting next year. Make sure you keep the area around them mulched and devoid of grass if possible, but keep the mulch from directly touching the trunk of the tree to avoid any rot issues.
Are your trees a particular cultivar or are they seedlings?