Section 9(3)&9(4)

Both section 9(3) and 9(4) cover cases where tax liability is to be discharged under reverse charge the only difference is as under: -

Under section 9(3) few goods and services are specifically notified by way of notification in which case tax is to be paid under reverse charge like in case of advocates, Goods transport agency etc. and -

Under 9(4) if supplies are received from unregistered dealer then tax is to be paid in reverse charges. (Though the RCM u/s. 9(4) has been deferred till 30.06.2018)
 

More details Refer::  gst-reverse-charge