H1B extension approved. About i94

[Otnip]

My present petition is expiring in August 2018. I have an approved extension till August 2021. I have planned an overseas trip not home country in July 2018. When I return there is still one month left on my present validity. Now my question is when I enter USA, will they issue me I94 upto August 2021 or upto August 2018. If they issue me upto August 2018, will I have to exit and reenter to have the new date stamped.
Please advise me so that I can make an informed choice.

[Otnip]

Please can someone help. If I am given i94 only till present validity, does it mean I will have to exit USA and re enter after getting the extended visa stamped.