https://esa.un.org/miggmgprofiles/indicators/files/Iran.pdf has slightly smaller numbers (albeit for 2012 and 2013): 868,172 refugees and 2,426,731 immigrants. (not clear whether refugees are included in the total immigrant population). Countries of origin are Afghanistan, Iraq, Pakistan, Azerbaijan and Turkmenistan. Shiites who find conditions in their home countries bad or intolerable, possibly?
::does some sniffing around::
yep.. as I thought. If you add all 3 together the total number of immigrants in China is 4,149,980.