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.