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H4 Extension / Change of status to H4

If you are filing for H4 extension or change your status to H4 as the dependent of an H1B employee, you should file FORM I-539. You must a copy of H1B holder's FORM I-94, H1B approval notice and evidence of your relationship (Example: Marriage certificate if spouse, Birth certificate if children)

From: http://www.h1bsponsors.com/html/h1bExtension.html