On the 1 floor, my Bai Niao has hatched three birds, but she laid another egg before leaving the nest. What should I do? Is it to kill that bird? Or let nature take its course

Ordinary eggs can be stored for about 10 days at room temperature, because your birds have hatched and nestled for about 20 days in the early stage, which is harmful to birds. If you continue to hatch, this egg may take 20 days, so I suggest you keep this egg for another week or so. If it doesn't continue, you can see how to deal with it yourself.