Can Green Card holder parents file F4 visa case for their married children?

[Tareque Mehdi]

Hi, my question is, my dad and mom have recently moved to the USA through family-based immigration F4 visa. They have already received Green Card. We are three brothers and all of us are married. Can my parents file a case for family-based immigration while having a green card? Or they need to first obtain the citizenship? What is the procedure? 

[JoeF]

Only US Citizens can sponsor married son's and daughters. That would be in family category F3. Same procedure as for F4.