Is it normal to sleep only 4 hours per night?

Not healthy. You need at least 7.5 to 8 hours of sleep. The effects of sleep deprivation and chronic lack of sleep:
Fatigue, lethargy, and lack of motivation
Moodiness and irritability
Reduced creativity and problem-solving skills
Inability to cope with stress
Reduced immunity; frequent colds and infections
Concentration and memory problems
Weight gain
Impaired motor skills and increased risk of accidents
Difficulty making decisions
Increased risk of diabetes, heart disease, and other health problems.