To help a 3 years and 9 months old child who has difficulty falling asleep at night, you can try creating a bedtime routine that includes calming activities like taking a bath, playing quietly, reading stories, singing soft songs, giving gentle massages, and using a night light if needed. It's also important to address any needs the child may have before bedtime to prevent multiple requests. You could also try waking the child up earlier in the morning, reducing screen time, and encouraging outdoor play before bed. These strategies have been successful for many parents in improving their child's sleep.