Hi Anitha,
The difference is between what is legal and what is practical.
Legally, as I understand, as long as the H4 is in the same status having current I-94, it is legal and in-status. One needs to apply for I-539 before the expiry of I-94.
Practically, H4 being dependent visa of H1, it may be good to be on the same page with the renewed & extended I-94.
As we know, the US has stopped issuing extension of H and other visas domestically, one has to go to home country of neighboring countries for those visas, if required. It is required then to have the extension approved.
DIGS
EB3, India
H1 / H4 Extension received in March 2005.