My four yr old son has a runny nose, with yellowish green mucus but no other symptoms, for about three days now, acting normal, what could it be?

Common cold: Your son is exhibiting common sypmtons of the common cold which you probably know is viral. Nasal discharge of different colors occur with any cold; it becomes signicant if the cold has lasted more than 10-14 days at which point it may indicate a bacterial infection. Then it will be a good time to visit his pediatrician.

Could be virus: Causing a sinus infection. These often go away on their own.. Saline nasal rinses can help..But if it continues for 5 days or so---or if other symptoms develop( fever cough)--call your dr. Allergies don't usually produce greenish mucus.