Two million will leave Syria as refugees, while the other 2.25 million will be displaced within Syria, according to a UN document seen by AFP Monday.
The conflict has already forced more than two million Syrians to flee, in a tide of humanity that has overwhelmed neighbouring countries. Millions more have been displaced inside Syria.
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Current estimates are that around 3.2 million Syrians will have become refugees by the end of this year in what Antonio Guterres, the United Nations High Commissioner for Refugees, has described as a "disgraceful humanitarian calamity".
The forecasts followed a meeting last month between humanitarian organisations and the UN's Office for the Coordination of Humanitarian Affairs (OCHA) in Amman.
OCHA said at the meeting that an "escalation of the conflict with the disruption of essential services and a further erosion of support mechanisms" was the most likely scenario for 2014, and warned of an increase in the threat to humanitarian workers.
The UN estimates that 8.3 million Syrians will need aid next year, putting further strain on already stretched resources.