What should i do about my 4-year-olds frequent nosebleeds?

Very common: Usually from allergies, picking on the nose, blowing nose too hard, constant rubbing etc. As long as there are no associated signs and symptoms like bruising , prolonged bleeding , cuts slow to heal, petechia or getting tired easily and he is healthy, then just treat the allergy. If + for above, need to see doctor to run test to rule out blood disorders.

Probably normal: This age group is hard because they are nose pickers. Many kids have frequent nosebleeds. In this age it is tough to get the nosebleed to stop completely because of the picking. Increasing the moisturizers (humidified air, moisturizing creams like eucerin, nasal saline sprays or gels) can be helpful. But these treatments are hard for this age group. If it continues to be a problem, see your doc.

More info....: As mentioned by others, most of the time it is not caused by bleeding disorder. However, if your son's nose bleeds are not only frequent, but long lasting (>20 minutes), i would recommend seeing your doctor. Also, if your son has any other symptoms like bleeding gums, bruising in "strange" areas of his body (such as chest, back, face), or bruises that have a hard lump (= purpura), see your doc.