Can H4 extension be done without entering to US ?


I got H1B visa valid till October 2017. Same is the validity for my wife (H4). I will be travelling to US in March. My employer will file for my extension before 6 months of expiry date i.e. somewhere in April. Wife will be travelling somewhere around July/ August.

Can I apply for her H4 extension, along with mine or H4 person needs to be in US for applying for extension?What is the minimum period required to apply for H4 extension ?