The U.N. refugee agency says this milestone comes barely 10 months after the 3 million mark was reached.
"This is the biggest refugee population from a single conflict in a generation," said U.N. High Commissioner for Refugees Antonio Guterres in a statement. "It is a population that needs the support of the world but is instead living in dire conditions and sinking deeper into poverty."
Of those who have left Syria, 2.2 million have been registered by the UNHCR in Egypt, Iraq, Jordan and Lebanon, 1.7 million in Turkey alone.