Normal variation in bowel movement frequency: In newborns, especially in the first few days of life, the frequency of bowel movements can vary widely. It's not uncommon for breastfed babies to have a bowel movement after every feeding, while formula-fed babies may have fewer. However, a 20-hour gap without a bowel movement in a 4-day-old could still be within the realm of normal, especially if the baby is otherwise asymptomatic.

Constipation: Although less common in newborns, constipation can occur, especially if the baby is formula-fed or has started on solids (though this would be unusual at 4 days old). Signs might include hard stools, straining during defecation, or obvious discomfort.
Dehydration: Newborns can quickly become dehydrated, which might lead to a decrease in bowel movements. This could be due to inadequate feeding, excessive vomiting, or other causes of fluid loss.
Hypothyroidism: Congenital hypothyroidism can cause constipation among other symptoms. While it's screened for at birth in many places, it's a consideration in a newborn with significant constipation.

Intestinal obstruction: Conditions like intestinal atresia, meconium ileus, or Hirschsprung's disease can cause a failure to pass meconium within the first 48 hours of life, which is a red flag. While the baby has already passed 4 days, any sign of obstruction (vomiting, abdominal distension) warrants immediate investigation.
Necrotizing enterocolitis (NEC): Although more common in premature infants, NEC can occur in term babies and presents with abdominal distension, vomiting, and sometimes blood in stool. It's a medical emergency.
Hirschsprung's disease: This congenital condition affects the colon and causes severe constipation or intestinal obstruction due to missing nerve cells in the muscles of the bowel. It's crucial to diagnose early to prevent complications.

Cystic fibrosis: While meconium ileus is a presentation of cystic fibrosis, it's relatively rare and typically associated with other signs such as respiratory symptoms.
Spinal cord anomalies: Conditions affecting the spinal cord, like spina bifida, can lead to neurogenic bowel, causing constipation or fecal incontinence. However, these would typically be identified prenatally or at birth.
Chromosomal abnormalities: Certain genetic conditions can increase the risk of gastrointestinal anomalies or functional issues, but these are broad and would likely be accompanied by other significant findings.